Knife Gate Valves

NOTE This standard covers the knife gate valve: a gate valve whose gate is a thin, beveled stainless steel plate drawn across the bore of a narrow body, built to MSS SP-81 or AWWA C520 in sizes 2 in. through 96 in., furnished for isolation of sludge, scum, grit, screenings, dewatered cake, slurry, and other solids-bearing or dry-solids service in water and wastewater treatment plants, pump stations, and industrial process facilities, in pipelines and on the outlets of the chutes, hoppers, and bins of solids handling equipment. (1.1)

NOTE The boundary of the work is the valve as a shop-assembled unit: the body and the bonnet where one is furnished, the gate, the seat and seals, the packing and gland, the stem, yoke, and superstructure, the end connections, the manual operator or the mounting interface for a power actuator, the shop coating, and the tests that prove the unit before it ships, together with the valve box and extension stem of a buried valve and the installation and field tests that prove the valve in the line. (1.2)

NOTE This standard is read with Common Work Results ProcessCommon Work Results for Process EquipmentResolves to the current adopted revision.sync/common-work-results-process, the umbrella for the process discipline, which carries the submittal register and procedures, the factory test witnessing policy, the manufacturer's field services, the equipment nameplates, the spare parts basis, the anchorage basis, and the vibration and noise limits for every process valve; nothing here restates them. (1.3)

NOTE A knife gate valve closes by drawing a thin plate with a beveled edge across the bore, so the gate shears a rag or a stringy solid rather than closing on it, and the body is a narrow casting with a seat at the bore and no pocket below the gate for solids to fill. An open bonnetless valve stores the gate in a chest above the bore that is open to the line fluid and sealed only by the packing the gate passes through, and a closed valve seals where the gate edge meets the seat and where the gate face meets the transverse seal at the packing. That construction is what makes the knife gate valve the isolation valve of screenings, grit, cake, and slurry lines and the valve on the outlet of a chute or a hopper, and it is also why the valve carries a lower pressure rating than a wedge gate or a plug valve, why its packing is adjusted and replaced over its life, and why its seal depends on the seat type and on the direction the pressure comes from. (1.4)

NOTE The eccentric plug valve under Eccentric Plug ValvesEccentric Plug ValvesResolves to the current adopted revision.sync/eccentric-plug-valves and the knife gate valve are the two isolation valves of solids service, and the valve schedule assigns each line to one of them. The plug valve carries the full AWWA C517 pressure rating, seals drip-tight in the direct-pressure direction and, where rated, in both, keeps that seal through years of cycling, and holds the line fluid inside a bolted cover behind a shaft seal rather than behind a packing gland. The knife gate valve passes rags and stringy solids that wrap a plug, cuts through grit that settles on a seat, weighs and costs a fraction of a plug valve in the large sizes, is built with the square or rectangular port that a chute or a hopper needs, and carries a pressure rating that falls with size, a packing gland that weeps and is adjusted, and a seat whose tightness depends on the seat type and on the direction of pressure. Neither is the valve of every sludge line, and this standard does not choose between them. (1.5)

NOTE MSS SP-81 describes the stainless steel, bonnetless, flanged knife gate valve of industrial and process piping in sizes 2 in. through 24 in., with its face-to-face dimensions, its pressure rating, its shell and seat tests, and the leakage it allows a metal seat. AWWA C520 describes the knife gate valve of the water and wastewater industry in sizes 2 in. through 96 in., in cast iron, ductile iron, carbon steel, and stainless steel bodies with metal or resilient seats, with a pressure rating assigned by size and its own shell, seat, and proof-of-design tests. A bonneted valve, whose gate retracts into a fully enclosed housing sealed at the stem rather than at the gate, is built to the body, gate, seat, and test requirements of whichever design standard governs it, with the bonnet added. (1.6)

NOTE The power actuator on a knife gate valve, whether electric under AWWA C542 or pneumatic or hydraulic under AWWA C541, with its integral controls, its open, closed, and position contacts, its position transmitter, and its network interface, is furnished under Valve And Gate ActuatorsValve and Gate ActuatorsResolves to the current adopted revision.sync/valve-and-gate-actuators, which also furnishes floor stands, bench stands, and the extension stems and stem guides of exposed valves operated from a floor above; the pneumatic cylinder that most knife gate valves ship with is that standard's product, mounted on the valve by the valve manufacturer. A knife gate valve with a V-port gate, or any knife gate that modulates in a control loop, is a control valve and is furnished under Control Valves And ActuatorsControl Valves and ActuatorsResolves to the current adopted revision.sync/control-valves-and-actuators. This standard furnishes the valve body such an actuator is mounted on, the mounting interface, and the thrust data the actuator is sized from. (1.9)

NOTE The isolation valves on the suction and discharge of a sludge, scum, or cake pump are furnished under this standard or under Eccentric Plug ValvesEccentric Plug ValvesResolves to the current adopted revision.sync/eccentric-plug-valves as the valve schedule designates for each valve, and the flushing and cleanout connections that Positive Displacement PumpsPositive Displacement PumpsResolves to the current adopted revision.sync/positive-displacement-pumps places between the pump and its isolation valves are furnished under that standard and the piping standards. (1.10)

NOTE The following are outside this scope and are governed by the companion standards named. (1.11)

NOTE Thermoplastic knife gate valves and sleeved plug valves for chemical service, steel gate valves under API 600 and API 602, and the screening and grit equipment, sludge thickening and dewatering equipment, and clarifier mechanisms whose chutes, hoppers, and bin outlets the valves of this standard mount on are not yet owned by a SynC standard. (1.12)

NOTE The drawing that shows the seat side and the stem orientation is the submittal that catches the most errors on this product, because a unidirectional knife gate valve that is correct in every material and rating still leaks when its seat faces the wrong way, and a valve whose chest fills with grit was installed in the orientation the drawing never showed. (3.1.4)

NOTE AWWA C520 requires a valve design to be proven once, by a cycling test under pressure followed by a leakage test, and allows a certified report of that earlier testing to stand for every later valve of the same design. Nearly every project accepts the report, because the test proves the design rather than the unit. Witnessing a fresh test on a contract valve is chosen where a design is new to the Owner, where the size is at the top of the manufacturer's range, or where a bidirectional rating or a pressure above the design standard's rating is being relied on. (4.1.2)

NOTE The valve manufacturer is the only party who knows the thrust its own gate needs to seat against the rated pressure and the thrust its gate and stem withstand, so the operator, the gearing, and the actuator stops are set where those numbers are known. A valve shipped bare and fitted with a cylinder in the field arrives with nobody responsible for the thrust match or the stroke. (4.2.3)

NOTE Size, pressure rating, port, seat type, and end connection are properties of each location on a project, and a standard that asserts any of them has decided the hydraulics, the solids, and the piping. The valve schedule carries them per tag, and this standard carries what every knife gate valve of a given design standard has in common. (5.1.3)

NOTE A knife gate held partly open puts the full velocity of a solids-bearing flow across the gate edge and the seat at the one place they seal, erodes the elastomer of a resilient seat in a season, and vibrates the gate in its guides. A valve that is closed and opened a few times a week lasts for years; the same valve throttled for a month needs a new seat. A knife gate that is meant to throttle carries a V-port gate and is a control valve under its own standard. (5.3.2)

NOTE Valves in exposed service are those installed in a building, a gallery, a vault, or above grade, where the operator is reachable by hand; valves in buried service are those installed in a trench and operated through a valve box. The datasheets of this standard carry a selection for each where the product differs between them. (5.4.1)

NOTE The chest of a bonnetless valve holds a pocket of line fluid above the bore that does not drain with the pipe, and the packing is the thinnest wall on the valve. Where that pocket freezes, the gate is locked in the chest and the packing is the part that splits, so the piping standard's freeze provisions reach the chest rather than stopping at the flange. (5.4.6)

NOTE Nearly every water and wastewater specification accepts a knife gate valve to either standard, because a manufacturer's catalog line in the sizes through 24 in. is usually certified to both and the two describe the same gate, the same seat, and the same tests, which is why the default admits both. Admitting AWWA C520 alone buys the full size range to 96 in., the iron and steel bodies, the resilient seat, and the water industry's proof-of-design test, and it is the choice of a utility that specifies by AWWA number. Admitting MSS SP-81 alone buys a stainless steel body on every valve and the industrial face-to-face dimension, and it is the choice of an industrial plant whose piping is laid out to that dimension. (6.1.3)

NOTE A bonnetless valve seals the gate where it leaves the body with a packing gland that is adjusted from outside, costs and weighs the least, and is the knife gate valve of nearly every sludge, grit, and chute application, at the price of a gland that weeps a little before it is adjusted and discharges onto the floor when it is neglected. A bonneted valve carries the gate inside an enclosed housing above the body and seals at the stem instead, so nothing reaches the atmosphere through the gate, it holds the full rated pressure in either direction with a seat on each side, and it is selected on digester and gas-side piping, on an odorous or hazardous fluid, on a valve that sees pressure from both sides, and wherever a weeping gland is not tolerated, at roughly twice the height and cost. (6.2.2)

NOTE MSS SP-81 rates its valves at 150 psi cold working pressure across the sizes it covers. AWWA C520 assigns a rating by size, with the full rating in the small and medium sizes and lower ratings in the large sizes, because a flat gate spans the whole bore and the thrust that line pressure puts on it grows with the square of the diameter while the gate stays a plate. Those ratings cover nearly every sludge, slurry, and chute line in a plant, and the field is derived from the size and the piping design pressure rather than fixed, so that a high-head sludge pump discharge or a cake line gets the heavier gate and body the manufacturers offer without a conversation about every other valve on the project. (6.3.4)

NOTE Line pressure that reaches the gate from the side opposite the seat presses the plate onto the seat, and the valve seals more tightly as the pressure rises. Pressure from the seat side lifts the plate off the seat, and the seal then depends on the gate guides, the transverse seal, and the operator's thrust, so the reverse rating of a unidirectional valve is a fraction of its direct rating. A bidirectional valve carries a seat on each side of the gate, or a perimeter seat that the gate is driven into from either direction, at a higher thrust and price. A valve that always closes against one direction, such as a tank or hopper outlet, is served by a unidirectional valve installed the right way round; a pump isolation valve that holds back the force main when the pump is off and holds back the pump when it runs against a closed valve, and any valve whose line is pressurized from either end for maintenance, needs the bidirectional rating. Neither is the norm across the services this standard covers, so none is defaulted. (6.4.4)

NOTE A standard round port is the bore of the valve as the design standards describe it, reduced from the pipe bore only by the seat, and it is the port of a knife gate valve in a pipeline. A full round port is the full nominal pipe bore through the valve with the seat recessed clear of it, and it is selected where the line is pigged or cleaned with a tool that has to pass through the valve, or where a cake line is sized tightly enough that the port reduction would raise the pressure drop at the valve. A square port and a rectangular port match the outlet of a hopper, a chute, or a bin, so that the whole opening is cleared when the gate is open and nothing bridges on a ledge, and they are the ports of a valve mounted on solids handling equipment rather than in a pipe. The port follows the line or the equipment at each location, so the valve schedule carries it. (7.2.2)

NOTE Gray cast iron is the body AWWA C520 is written around and the body of most knife gate valves in municipal sludge and grit service, at the lowest cost and in the full size range, with its wetted surfaces behind an epoxy coating. Ductile iron carries roughly twice the strength and a ductility that gray iron lacks, and it is selected where the valve is rated above the standard pressure, where a lugged body is bolted to a steel flange and the bolting load would overstress a gray iron lug, or where the valve is buried in a line subject to ground movement. Carbon steel is the body of a valve rated for a pressure or a size the iron bodies do not reach, and of an industrial plant whose piping is steel. Type 304 stainless steel is the body MSS SP-81 requires and the body of a chemical, food, pulp, or chloride service where a coated iron body would be cut through at the first holiday, and Type 316 adds resistance to chlorides and is selected in a coastal plant, on digested sludge with a high chloride content, and wherever the plant standardizes on it. No material is the norm across the municipal and industrial services this standard covers, so none is defaulted. (8.1.2)

NOTE A wafer body is sandwiched between the piping flanges on through bolts that pass outside the body, is the lightest and least costly knife gate body, and depends on both flanges being in place to hold it, so it cannot be left at a dead end or have its downstream pipe removed under pressure. A lugged body carries tapped lugs that take a separate bolt from each flange, so either flange is unbolted with the other side under pressure and the valve serves at a dead end, on a hopper outlet, and anywhere the downstream piping is dismantled for service; it is the body most sludge and slurry specifications call for. A flanged body carries a full flange on each face, drilled through, and it is the body of the large sizes and of AWWA C520 valves bolted to flanged ductile iron. Grooved ends join to grooved piping under its coupling where the piping system is grooved throughout, and threaded ends are available only in the smallest sizes for a threaded drain or seal water line. The connection follows the piping at each location, so the valve schedule carries it. (8.2.2)

NOTE The Class 125 iron template and the Class 150 steel template share one bolt circle and bolt count through 24 in., so a single drilling bolts a knife gate valve to the flanged ductile iron, cast iron, and steel piping of a plant and matches the rating the valve carries, which is why that drilling is what the valve is bolted to on nearly every project. The Class 250 and Class 300 template is selected where the connected piping is drilled to it for a pressure above what the Class 125 flange carries, and the heavier flange needs the heavier body behind it. The AWWA C207 Class D template matches the fabricated steel flanges of large waterworks piping, which share the Class 125 pattern in most sizes and diverge in a few, and it is selected where a large valve bolts to AWWA C207 flanged steel pipe. (8.3.3)

NOTE Plated carbon steel is the bolting an iron valve ships with and is adequate in a dry, heated gallery where it is inspected and painted with the valve. Type 304 holds up in the damp, hydrogen sulfide atmosphere of a sludge gallery or a headworks, where plated steel rusts within a few years and a seized yoke bolt turns a packing change into a valve change. Type 316 adds resistance to chlorides and is selected in a chlorinated gallery, near the coast, or wherever the plant standardizes on it. The gland bolting is stainless on every valve because it is the bolting a crew turns most often and the one that sits in whatever the packing weeps. The rest of the decision follows the atmosphere the valves sit in, which the project knows and the standard does not. (8.4.5)

NOTE The gate is the part that cuts the solids, bears on the seat, slides through the packing, and corrodes at the line where the fluid meets the air. Type 316 stainless steel is the gate nearly every manufacturer ships as standard for municipal sludge, grit, and slurry and the one that resists the chlorides and sulfides of wastewater for the life of the valve, which is why it is the default. Type 304 costs a little less and is selected where the fluid is clean water or a non-chloride slurry and the Owner standardizes on it. Type 317 and duplex stainless steel are selected for a chemical or a high-chloride service, such as a brine, a digested sludge with a high chloride content, or a coastal plant, where Type 316 pits. A precipitation-hardened gate resists the gouging of a coarse abrasive slurry and holds its edge on a hard solid. A hard-faced seating edge, overlaid with a chromium carbide or a similar alloy on a Type 316 plate, is selected for grit slurry, sand, and other abrasive services where a plain stainless gate is cut at the seat in a year. (9.1.2)

NOTE A gate that leaves the packing with a scored face brings a leak back through it on every stroke, and the sharpness of the beveled edge is the difference between a valve that cuts a rag and a valve that folds the rag over the seat and never closes again. The gate of a knife gate valve is a wearing part, and the material is matched to what wears it. (9.1.6)

NOTE A metal seat is the stainless steel seating surface machined into the body or retained in it as a ring, seals to the leakage MSS SP-81 and AWWA C520 allow a metal seat rather than drip-tight, tolerates the temperature and the abrasive solid that would cut an elastomer, and is the seat of a grit slurry, a hot or chemical line, and a dry solids chute where a small leakage is of no consequence. A resilient perimeter seat is an elastomer ring around the bore that the gate edge is driven into, seals drop-tight at the rated pressure and, where the design carries it, in both directions, and is the seat of a sludge, scum, and cake line where the leakage of a metal seat would be a wet floor or a pump that will not prime; it wears where the gate edge strikes it and is replaced as a ring. A polymer seat of PTFE or a similar material seals tightly against a chemical or a food-grade fluid that attacks an elastomer and carries a higher temperature, at the price of a harder seat that takes a set under a long closure. The seat follows the fluid and the leakage the service tolerates at each location, so the valve schedule carries it. (9.2.2)

NOTE The elastomer is matched to the fluid first. EPDM resists chlorine, chloramine, ozone, hot water, and the sulfides of wastewater, which makes it the usual choice on sludge, scum, and plant water, and it is attacked by petroleum oils and grease. Nitrile resists oils, grease, and hydrocarbons and is selected on scum, on an industrial sewer that carries them, and on a cake line dosed with a polymer carried in oil. Neoprene sits between the two on oil resistance and is specified where a utility's stock is neoprene. Natural rubber has the best abrasion resistance of the group and is selected for grit slurry at the cost of ozone and oil resistance. Fluoroelastomer carries the broadest chemical resistance and the highest temperature rating at the highest price and is selected for a chemical or a hot line. Chlorobutyl resists ozone, acids, and the permeation of gas and is selected on digester gas-side piping. No compound is the norm across the services this standard covers, so none is defaulted. (9.2.7)

NOTE PTFE-impregnated synthetic braid seals a smooth stainless gate with a low drag, takes an adjustment without hardening, and holds up in sludge, slurry, and plant water to the temperature of any municipal line, which is why nearly every manufacturer ships it as standard and why it is the default. Graphite braid carries a line temperature that PTFE does not and is selected on a hot industrial line. Aramid braid resists the extrusion and the abrasion of a packing that a coarse slurry works into, at a higher gate drag, and is selected on grit and sand. An elastomer transverse seal with braided packing above it puts a molded seal against the gate face as the primary barrier and keeps the packing as the backup, and it is the arrangement of a valve meant to run with no weep, as on a bonnetless valve in a finished room. (9.3.2)

NOTE The packing is where a bonnetless knife gate valve leaks to the outside, and the leak is sludge on the gallery floor. A gland that is adjusted from outside, with the valve in service, is tightened on the day the weep is noticed; a gland that needs the yoke pulled is tightened the day the line is drained for something else, which on a sludge line is a long way off. The stuffing box depth is what gives the packing enough rings to be taken up more than once before it is replaced. (9.3.6)

NOTE Chest flush ports are tapped connections into the body chest above the bore, one on each side of the gate, through which plant water is admitted to wash the solids out of the chest before the valve is closed and to clear the packing area on a line that has stood idle, and they are furnished on nearly every valve in sludge and slurry service, which is why they are the one provision defaulted. A deflection cone or a splash guard is fitted below the body of a valve whose seat design discharges a small volume of line fluid past the gate on each stroke, or whose gland is above an open floor, so that the discharge is directed to a drain rather than onto the floor or a person, and it is selected per valve by the arrangement below it. A replaceable body liner of elastomer or a hard alloy protects the body of a valve in abrasive slurry where the bore itself would be worn, so that the liner rather than the casting is replaced. A gate scraper at the packing wipes the gate face as it retracts, so that cake, grease, and fibrous solids are not drawn into the packing and the chest, and it is the provision of dry solids and cake service. A chest purge connection for air serves a valve on a dry solids chute or hopper, where water would turn the dust in the chest to a paste, and it admits plant air to clear the chest instead. (9.4.2)

NOTE A rising stem lifts with the gate, shows the gate position at a glance from across the gallery, keeps its threads dry and greased above the packing, and is the stem of nearly every knife gate valve made, which is why it is the default. A non-rising stem turns in place inside a yoke sleeve and needs no clearance above the handwheel, and it is selected where the valve sits under piping, in a pit, or at a chute with no headroom, at the price of threads that run inside the yoke and a separate position indicator. (10.1.2)

NOTE The stem carries the full operator thrust into the gate and, on a rising stem valve, stands above the packing in whatever the gallery air holds. Type 304 is the stem most manufacturers ship as standard and serves a water plant gallery and a dry solids building. Type 316 is selected in a sludge or headworks gallery with hydrogen sulfide, near the coast, and wherever the plant standardizes on it, so that the thread the operator turns against does not pit. Neither is the norm across the plants this standard covers. (10.2.2)

NOTE A coated cast iron or carbon steel yoke is the superstructure most iron-bodied valves ship with, and its coating is what stands between it and the gallery atmosphere, so it is the yoke of a dry, heated gallery and of a valve that is painted with the piping. A ductile iron yoke carries an actuator and the thrust of a large valve with a margin that cast iron lacks and is the yoke of a power-actuated or a large valve. A stainless steel yoke is the yoke MSS SP-81 valves carry and the one selected wherever the body is stainless, wherever the atmosphere is corrosive, and on a valve that is washed down, at the highest cost. The yoke follows the body and the atmosphere, which the project knows and the standard does not. (10.3.2)

NOTE The screening, grit, thickening, and dewatering equipment the valve mounts on is not yet owned by a SynC standard; the interface between the valve and the equipment outlet is the chute and hopper mounting drawing submitted under this standard, which shows the outlet opening, the flange, and the column of material above the valve. (11.6)

NOTE A knife gate on a hopper outlet closes through a column of screenings or cake that has been pressing on the open gate since the bin was filled, and the thrust to drive the gate through that column, not the line pressure, is what sizes the operator. Water in the chest of such a valve turns the dust to a paste that locks the gate, which is why the chest is purged with air; a scraper at the packing is what keeps the cake from riding into the stuffing box on every stroke. (11.7)

NOTE A handwheel on the stem is the operator of nearly every exposed knife gate valve at an operating floor, because the valve is a multi-turn valve whose stem thread already multiplies the operator's effort, which is why it is the default. A handwheel fitted with a nut lets the same valve be operated by hand at the floor and by a portable actuator when the pull is heavy. A 2 in. square nut in place of the handwheel suits a valve in a pit or a chamber that is reached with a key, and it removes a handwheel that would be in the way. A chainwheel replaces the handwheel where the valve is overhead and is reached from the floor by a chain. A bevel-gear operator is the operator of the large sizes, where the stem thrust at the rated pressure exceeds what a person applies through a handwheel, and the Gear Operators article sets the size at which it is furnished regardless of this selection. A lever is a quick-acting operator available only on the smallest sizes, where the thrust is low enough to push the gate home by hand, and it suits a sample line or a small drain. (12.1.3)

NOTE The stem thrust to seat a knife gate rises with the square of the valve size and with the rated pressure, and in sludge and slurry service it rises again with the drag of the packing and the solids the gate cuts, so that somewhere between 10 in. and 16 in. it exceeds what a person applies through a handwheel within the rim pull limit. Utilities set the threshold their crews work to, and manufacturers fit gearing as standard from a size that differs between them, which is why the size is not defaulted. A lower threshold buys an easier valve at the cost of a higher turn count and a gear case on every mid-size valve. (12.2.2)

NOTE Eighty pounds is the rim pull the AWWA valve standards have long used as the limit a person can apply to a handwheel, and it is the figure nearly every waterworks specification carries, which is why it is the default. A lower limit buys an easier valve at the cost of a higher gear ratio and more turns from open to closed, and it is selected where the operators work from a ladder or a platform, or where a plant standardizes on a lower figure. (12.3.2)

NOTE Operators sized to the rated pressure rather than to the project's operating pressure are what let a valve be closed on the day the pressure is at its highest, which is the day a pump runs against a plugged line and the full shutoff head is on the closed gate. An operator sized to the normal operating pressure is sized to the one condition under which nobody needs to close the valve. (12.3.6)

NOTE Most North American utilities open their valves counterclockwise, and a plant whose valves open the other way from the ones around them has an operator closing a valve that was meant to be opened on the day it matters. A minority of utilities, often whole regions, open clockwise, and a project in their territory follows the owning utility's practice without exception. The direction is set once for a project and is cast on every operator. (12.5.2)

NOTE Pneumatic and hydraulic cylinder actuators under AWWA C541 and electric motor actuators under AWWA C542 on knife gate valves under this standard, with their solenoid valves, limit switches, position transmitters, integral controls, open, closed, and position contacts, and network interface, are furnished under Valve And Gate ActuatorsValve and Gate ActuatorsResolves to the current adopted revision.sync/valve-and-gate-actuators, and the actuator on a knife gate valve that modulates in a control loop is furnished under Control Valves And ActuatorsControl Valves and ActuatorsResolves to the current adopted revision.sync/control-valves-and-actuators. This standard furnishes the valve, the yoke the actuator mounts on, the mounting interface, and the thrust data. (13.2)

NOTE A pneumatic cylinder is the actuator most knife gate valves ship with, because the valve is a linear-stroke valve and a cylinder on the yoke drives the gate directly with no gearing, at low cost, from the plant air that a solids handling building already carries, and it is the actuator of a chute, a hopper, and a sludge line that is cycled often. A hydraulic cylinder delivers the thrust of a large or a high-pressure valve in a shorter cylinder and is selected where a plant carries a hydraulic power unit for its gates. An electric motor actuator drives the stem through its own gearing from the plant's electrical supply, holds its position without a supply, and is selected where no plant air reaches the valve, where the valve is operated from the control room a few times a year, or where the Owner standardizes on electric actuation across the plant. (13.3)

NOTE A cylinder that stalls at its full supply pressure puts its whole thrust into the gate, the stem, and the yoke, and a cylinder sized with a generous margin for a packing that was later tightened bends the stem on the first close against a solid. The valve manufacturer is the only party who knows what the yoke and the gate withstand, and the written confirmation is what puts that number in front of the actuator supplier before the regulator is set. (13.9)

NOTE Fusion-bonded epoxy is applied as a powder to a heated casting and cures into a hard, holiday-free film that bonds to blast-cleaned iron better than any liquid coating and resists the abrasion of grit, and it is the coating of most iron-bodied knife gate valves. Two-part liquid epoxy to the same standard is applied by spray in any coating shop and is the system a manufacturer uses where its castings or fabrications are too large to heat. Both meet AWWA C550, and the choice follows the manufacturer's process rather than the project. (14.1.2)

NOTE The shop epoxy is a complete corrosion barrier on its own and is the finished exterior on a valve in a vault or a gallery where nobody looks at it. In a plant whose piping is color-coded or finished to a scheme, the valve exterior is topcoated in the field to match the pipe, and the shop epoxy becomes the primer that topcoat has to bond to. The decision follows the plant's finish scheme rather than the valve. (14.2.2)

NOTE A knife gate valve is buried rarely, because its packing gland is adjusted and its chest is flushed from outside the valve, and neither is reached through a valve box. Where a sludge or slurry main between structures carries one, the valve is set in a vault, or it is a bonneted valve the manufacturer rates for burial, with its operating nut brought to grade, and the choice is made per valve. (15.2)

NOTE A valve box is the lowest-cost access, a cast iron shaft from the operator to a lid in the surface, and a crew operates the valve from grade with a key; it suits a bonneted valve the manufacturer rates for burial that is operated a few times a year and needs no gland adjustment. A vault puts the operator, the gland, the chest flush ports, and the flanges where a person can reach them, and it is selected for a bonnetless valve, for a valve that is operated often, for a valve that carries a power actuator, and wherever a leak at the gland has to be seen before it reaches the ground, at the cost of the structure and the confined-space entry it creates. (15.4)

NOTE An operating key reaches a nut a few feet down without trouble and loses its purchase as the depth grows, and a crew that cannot find or seat the key on a deep nut leaves the valve as it found it. Utilities set the depth their crews work to, commonly within a few feet of grade, and a deeper main gets an extension stem that brings the nut up. The depth is the utility's practice rather than the valve's, which is why it is not defaulted. (15.7)

NOTE The extension stems, stem guides, and floor stands that operate exposed valves from a floor above are furnished under Valve And Gate ActuatorsValve and Gate ActuatorsResolves to the current adopted revision.sync/valve-and-gate-actuators; this section covers extension stems inside valve boxes on buried valves only. (15.11)

NOTE MSS SP-81 sets the shell test at one and a half times the rated working pressure and the seat test at the rated pressure, and AWWA C520 prescribes its own shell and seat pressures and durations by size. The seat test is performed in the direction the valve is rated for because a knife gate seals by the pressure pushing the plate onto the seat, and a valve rated in both directions is tested in both. A gate that seals in one direction and weeps in the other is sitting off center in its guides or has a transverse seal that was pinched on assembly, and the test finds it before the gallery floor does. (16.4)

NOTE A knife gate valve lifted by its yoke carries the full weight of the body through the yoke bolts and the stem, and the gland and the stops that were set at the factory are the first things to shift. The gate is shipped closed because the beveled edge is protected in the seat, and because a gate left standing above the body in a laydown yard is bent by the first thing that is set against it. (17.4)

NOTE A pump discharge isolation valve is closed so the pump can be serviced, and while it is closed the pump is off and the force main's static head is on the far side of the gate. Orienting that valve for the running condition, with the pump pressure pressing the gate onto the seat, seals it while the pump runs and lets it weep onto the open pump while the mechanic works on it. The closed condition that matters is the one the valve is closed for. (18.1.2)

NOTE With the stem up, the operator stands above the pipe where it is reached and read from the walkway, the chest drains back into the bore through the open gate, the gland and the flush ports are at hand, and the gate closes downward through whatever has settled in the invert and cuts it, which is the orientation nearly every knife gate valve is installed in and why it is the default. With the stem horizontal, the gate closes sideways across the bore and the chest sits beside the pipe rather than above it, so a settled solid is pushed aside rather than closed upon and the chest is out of the path of the solids that a turbulent flow throws upward into it, at the cost of a side-mounted operator that needs a gear and a support and a chest that no longer drains by gravity. With the stem down, the chest hangs below the pipe and collects whatever settles, the packing sits in the sediment, and the valve is installed that way only where headroom leaves no alternative and the manufacturer rates the valve for it. (18.1.4)

NOTE A cast iron body bends across the edge of a misaligned mating flange as the bolts are tightened, and cast iron has almost no ductility to absorb the bending. The crack appears at the lug or the flange, and the valve is blamed for a piping error. A wafer body that loses one flange loses the bolts that hold it in the line. (18.2.4)

NOTE A valve that forms a piping test boundary is being asked to hold the test pressure on its gate, and the test pressure of the piping is set above the valve's rating by design. A unidirectional valve with the test on the wrong side of the gate leaks, the test fails, and the valve is blamed; a valve with the test above its rating in any direction is damaged by the test it was never rated for. A blind flange is the test boundary, and the valve is tested open inside it. (18.3.4)

NOTE The Engineer of Record witnesses the test on most projects because the test record is the Engineer's evidence that the valve matches its submittal, which is why that witness is the default. An Owner's operator witnesses where the plant staff will operate the valves and wants to see each one work before it is handed over, which is common on a plant expansion. A contractor's quality control representative witnesses where the contract carries a quality control program and the Engineer reviews its records rather than attending. (18.3.11)

NOTE The cost that goes unassigned in a valve warranty is never the valve. It is the outage to drain the sludge line it sits in and the cleanup of what a failed gland put on the floor. Naming those costs, and drawing the line between the packing adjustment that is the Owner's and the packing failure that is the Contractor's, is what makes the warranty worth having on a valve whose one wearing seal is adjusted by hand. (19.6)

NOTE The packing and the transverse seal are the two wearing seals every knife gate valve has and the ones a crew replaces in place, so a set per size is the spare that is used, which is why they are the two defaulted. A resilient seat is replaced through the port when the gate edge has cut it, and holding one per size keeps a sludge line from waiting on a molded part. A replacement gate for a large valve is a long-lead plate that is worth holding where the valve isolates something the plant cannot do without. Operating keys are furnished because a buried valve without a key within reach is a valve nobody can close. (20.3)