Plug Valves: Lubricated vs Non-Lubricated Types, Ports and Selection

Quick Answer

A plug valve is a quarter-turn valve whose closure is a tapered or cylindrical plug with a port through it: a 90° turn lines the port up with the pipe or turns solid metal across the flow. The plug seals over a far larger area than a ball, which suits dirty, sticky and abrasive fluids but raises operating torque. In a lubricated plug valve, injected sealant fills the clearance between plug and body, sealing and lubricating it, and must be topped up on a schedule. Non-lubricated valves seal against a PTFE sleeve, a fluoropolymer lining or an elastomer sleeve, or lift the plug off its seat before turning it. Eccentric plug valves swing an offset plug onto its seat and lift it clear on opening; they are the wastewater type, covered by AWWA C517 and MSS SP-108. Expanding plug valves give double block and bleed. API 599 covers metal plug valves from NPS ½ to 36, Class 150 to 2500, in four port patterns: short, regular, venturi and round-port full-bore; only the last passes a rigid pig. Flow through a multiport valve's branch meets about five times the resistance of a two-way plug valve.

A typical call from a sewage pumping station: every time the pumps stop and the automated discharge valves close, the force main drains back into the wet well. The valves are new eccentric plug valves, but they were fitted with the seat end facing away from the pumps, so with the pumps off the force main's head pushes the plug off its seat instead of into it.

Plug valves have one moving part and turn through 90°, but type, port, sealing method and even the direction the valve faces decide whether it works. This guide covers each type, the standards, the key numbers and what to send for a quotation.

How a plug valve works

A plug valve has three main parts: the body, the plug, a tapered or cylindrical block with a port through it, and the stem that turns it. It is one of the oldest valve designs, still called a cock in some industries.

The difference from a ball valve is the sealing surface. A ball seals on two narrow seat rings; a plug seals over a wide band of body round each port. That band wipes the seat as the plug turns, in most designs leaves no open cavity for solids to settle in, and stays tight in dirty service. The price is friction: more seal area means more torque.

A tapered plug is held into its taper by an adjuster or gland: too tight and it binds, too loose and it leaks. Pressure surges and the plug's own weight can drive it deeper until it locks. Many lubricated designs counter this with sealant injected under pressure, which pushes the plug back toward the large end of its taper; others use springs, stops or a pressure-balanced plug.

Lubricated plug valves

How the sealant works

Sealant is injected under pressure through a fitting on the stem or body into grooves around the ports. It fills the clearance between plug and body, forming the seal and the lubricating film at the same time. A fresh charge keeps torque low and can often stop a valve weeping.

The sealant is a consumable: it washes out, dries or hardens, and the valve then turns stiffly and starts to pass. One lubricated plug valve maker's re-sealing guidance:

The maker treats these intervals as approximate and says to re-seal two-way valves only fully open or fully closed (multiport valves at the ends of travel), and to re-seal and cycle valves stored for three months or more before use.

Choosing the sealant

Use only a sealant the valve maker approves for the service. One that dissolves in the product stops sealing; one that hardens at the operating temperature seizes the plug. Sealant gradually washes out into the process, so lubricated valves are a poor choice where product purity matters.

Non-lubricated plug valves

• Sleeved: a PTFE or other polymer sleeve locked into the body seals all round the ports, tight in both directions without sealant.
• Fully lined: the body bore and plug are lined with a fluoropolymer such as PTFE or PFA, so no metal touches the fluid; a common choice for corrosive chemicals.
• Elastomer-sleeved: an EPDM, NBR or FKM sleeve, chosen for the fluid and temperature.
• Lift-type: the plug rises off its seat before it turns and drops back afterwards, so the sealing faces never slide under load.
• Expanding plug: seal segments retract before the plug turns, then wedge outward against both body seats, with a bleed between them. This is the classic double block and bleed valve for meter provers, tank farms and product segregation.

Rate the polymer, not just the body: the maker's pressure–temperature chart for the sleeve or lining governs.

Eccentric plug valves for wastewater and slurry

In an eccentric plug valve the seat is offset from the shaft. The plug, usually with a resilient facing, swings across the flow and meets the seat only at the end of its travel; on opening, it lifts off as it rotates. With no rubbing, grit does little damage, and the further the plug is closed, the tighter it seals, so a worn valve can often be closed a little further to restore shut-off.

• Ports: rectangular or round, with standard 80% port or full 100% port openings; both can be cleaned with flexible urethane pigs
• Seats and linings: metal-to-metal seats for severe duty such as activated sludge; rubber or glass linings for abrasive fluids

Standards. Eccentric plug valves were first standardised in MSS SP-108; AWWA C517 followed for water utilities. Its 2016 edition covers 75 mm (3 in) to 1,800 mm (72 in) valves for water, wastewater and reclaimed water at 0.6–52 °C (33–125 °F), with a minimum design pressure of about 12 bar (175 psig) up to 300 mm (12 in) and about 10 bar (150 psig) from 350 mm (14 in), and a maximum full-open velocity of 2.4 m/s (8 ft/s) on the nominal valve size. It sets no minimum waterway area or head-loss limit, so compare makers' port and flow data.

Velocity check. A DN 200 valve on a force main carrying 70 l/s runs at 0.070 ÷ (π/4 × 0.2²) = 2.23 m/s, inside the 2.4 m/s rating. At 80 l/s it reaches 2.55 m/s; a DN 250 valve brings that down to 1.63 m/s.

Installation — the rules the pumping station in the opening ignored:

• Seat end toward the pump, so that when the pump stops, system pressure pushes the plug into the seat (the preferred, direct pressure direction)
• Horizontal lines with settling solids: shaft horizontal, so the open plug sits at the top of the pipe, clear of the solids
• Vertical lines: seat end up, so solids do not collect in the closed valve

Eccentric valves seal against reverse pressure too, with more closing rotation, and handle moderate throttling. For the pumps, see our guide to sewage and wastewater pumps.

Plug valve types compared

Ports, patterns and pressure loss

Most plug valves have rectangular ports; round ports give a full circular bore. API 599 names four body patterns:

Only round-port full-bore valves pass rigid pigs (flexible foam pigs can pass some reduced ports, as in eccentric valves); see our guide to pipeline pigging.

Worked example: what the flow path costs

A DN 200 (8 in) Schedule 40 line, bore 202.7 mm, carries 250 m³/h of water (about 1,100 US gpm).

• Flow area: π/4 × 0.2027² = 0.0323 m², so velocity = 0.0694 m³/s ÷ 0.0323 m² = 2.15 m/s
• Velocity head: ρv²/2 = 998 × 2.15² ÷ 2 ≈ 2.31 kPa
• Standard equivalent-length data give K = fT × L/D, where fT, the fully turbulent friction factor of clean steel pipe, is 0.014 for 8 in pipe (the Colebrook equation reproduces it)

Cv = 29.9 d² ÷ √K with d = 7.981 in, and Kv = 0.865 Cv. As a check, the plug valve's loss from its Cv is (1,101 ÷ 3,794)² = 0.084 psi, or 0.58 kPa, matching the K method.

1. The flow path matters more than the valve type. The 3-way branch path has three times the resistance of the same valve's straight-through path, five times that of a two-way plug valve and 30 times that of a full-bore ball valve.
2. Small losses count where head is scarce. On a pumped discharge 2.9 kPa is negligible; on a pump suction it is 0.30 m of water, a real share of a tight NPSH margin (see pump curves and system curves).
3. Generic figures are a starting point. Reduced-port patterns lose more; compare makers' Kv or Cv data.

Multiport plug valves

A 3-way or 4-way plug valve replaces two or three valves and a tee in tank switching, pump changeover or duty/standby service. Its port arrangement, the flow plan, decides which ports connect in each position, and flow plans differ between makers. Specify:

• Each position: which ports connect and which are closed, as a sketch
• Mid-stroke behaviour: whether the new path opens before the old one closes; on a pump discharge or relief valve changeover, a moment with every path shut is not acceptable
• Shut-off: which closed ports must hold tight, and against what differential pressure

API 599 does not cover 3-way and 4-way valves, so put the rating, testing and flow plan in the order, and check the flow plan marks at installation.

Standards for plug valves

Check the current edition, or the one your project specification names.

Torque and actuation

Plug valves need more torque than ball valves of the same size because the whole seal band slides as the plug turns. Breakaway torque, to start a valve that has stood closed, exceeds running torque; it rises with differential pressure and time standing, changes with temperature, and in lubricated valves depends on the state of the sealant. Eccentric valves are the exception: friction stays low until the plug meets the seat.

MSS SP-91 takes a typical operator as able to apply about 670 N (150 lbf) on a 300 mm (12 in) lever at waist height, or about 200 N·m (150 lbf·ft). A valve that needs more needs a longer wrench, a gear operator or an actuator. For actuators:

• Size on the maker's breakaway torque at maximum differential pressure, not running torque
• Add the margin your specification or the actuator maker requires
• On lubricated valves, keep up the sealant schedule: dry sealant raises torque

For actuator types and mounting, see our guide to pneumatic vs electric valve actuators.

Plug valve vs ball valve

Choose a plug valve for dirty, viscous, abrasive or sticky fluids, for multiport switching, or for an expanding plug's double block and bleed. Choose a ball valve for clean service, lower torque and fast automated isolation. Neither is a precise control valve; for that, see control valves and Cv sizing.

Common Mistakes

1. Never re-sealing a lubricated valve. The sealant dries, torque climbs and the valve sticks or leaks when next needed.

   Prevention: Put sealant injection on the maintenance plan at the maker's interval.

2. Using the wrong sealant. A sealant that dissolves in the product stops sealing; one that hardens seizes the plug.

   Prevention: Use only the sealant the maker approves for the fluid and temperature, and record it on the valve tag.

3. Fitting an eccentric valve the wrong way round. With the seat end away from the pump, the static head pushes the plug off its seat whenever the pump stops.

   Prevention: Seat end toward the pump; shaft horizontal in solids service; seat end up in vertical lines.

4. Putting a reduced-port valve in a pigged line. A rigid pig cannot pass a short, regular or venturi pattern port.

   Prevention: Specify round-port full-bore and check the bore against the pig.

5. Ordering a multiport valve without a flow plan. The ports connect the wrong way, or every path shuts mid-stroke on a pump discharge.

   Prevention: Send a sketch of each position and the mid-stroke behaviour needed.

6. Sizing the operator on running torque. The valve will not break away after standing closed.

   Prevention: Use the maker's breakaway torque at maximum differential pressure, plus your specification's margin.

7. Throttling a standard plug valve. It controls poorly, and high-velocity flow at a part-open port wears the sealing surfaces.

   Prevention: Use an eccentric plug valve for moderate throttling in wastewater, and a control valve for precise control.

Ordering checklist

• Service: fluid, solids, temperature range, design pressure, maximum differential pressure across the closed valve
• Type: lubricated, sleeved, lined, lift-type, eccentric or expanding plug
• Size and rating: NPS or DN, class or PN, body material
• Port: pattern, or percentage port for eccentric valves; pigging requirement
• Ends: flanged (with facing), butt-welding, threaded, socket-welding, grooved or mechanical-joint
• Multiport: flow plan for each position, mid-stroke behaviour, tight shut-off ports
• Operation: wrench, gear operator or actuator, with supply, fail position and mounting
• Standards: API 599, API 6D, AWWA C517, MSS SP-108 or the project specification, and the approved sealant for lubricated valves
• Testing and documents: shell and seat tests, witness points and certificate type; see our guide to material test certificates
• Commercial: quantity, destination and required date

Supply from Kasko Makine

Kasko Makine supplies plug valves:

• Plug valves — quarter-turn shut-off valves with a tapered or cylindrical plug and a rectangular or round flow port, in lubricated or non-lubricated construction
• Materials: cast iron, carbon steel, stainless steel and bronze
• Applications: on-off flow control, flow diversion with multi-port valves, moderate throttling, and oil, gas, chemical and water services

Engineering support: Send the fluid, temperatures, pressures, size, class, ends and operation, plus a flow-plan sketch for multiport valves. We will review the valve type, port and sealing method against your data.

Send the fluid, size, pressure class, end connections and operation for a plug valve proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Double Block and Bleed Valves: DBB vs DIB, Designs and Testing
• Ball Valves: Floating vs Trunnion, Types and Selection
• The Complete Guide to Industrial Valves
• Sewage and Wastewater Pumps: Types and Selection
• Control Valves and Actuators: Cv Sizing and Selection
• Pneumatic vs Electric Valve Actuators and NAMUR Mounting

Frequently Asked Questions

Q: What is a plug valve and how does it work?
A: A plug valve is a quarter-turn valve with a tapered or cylindrical plug in the body. A port runs through the plug: a 90 degree turn lines it up with the pipe to open the valve, or turns solid metal across the flow to close it. The plug seals over a wide band around each port, which suits dirty fluids but needs more torque than a ball valve.

Q: What is the difference between lubricated and non-lubricated plug valves?
A: In a lubricated plug valve, sealant injected through fittings fills the clearance between plug and body, where it seals and lubricates. It needs a sealant approved for the fluid and regular re-sealing. Non-lubricated valves seal against a PTFE sleeve, a fluoropolymer lining or an elastomer sleeve, or lift the plug clear before turning. They need less maintenance but are limited by the polymer.

Q: How often should a lubricated plug valve be re-sealed?
A: Follow the valve maker's interval. One maker's guidance is daily for valves operated 10 to 100 times a day, weekly for one to ten times a day, quarterly for one to 30 times a month, and twice a year for valves used less often. Re-seal two-way valves fully open or fully closed and multiport valves at the ends of travel, and re-seal valves stored for three months or more before use.

Q: What is an eccentric plug valve used for?
A: Eccentric plug valves are used mainly in water, wastewater, sewage and sludge service. The seat is offset from the shaft, so the plug lifts clear as it opens and meets the seat only at the end of closing, which limits wear from grit. AWWA C517 and MSS SP-108 cover the resilient-seated cast-iron type. Install it with the seat end toward the pump.

Q: What are the port patterns of a plug valve?
A: API 599 names four patterns. The short pattern has the face-to-face dimension of a comparable flanged gate valve and is limited to Class 150 and 300. The regular pattern has a larger port than the venturi pattern, usually in a longer body than the short pattern. The venturi pattern has a reduced port with a venturi-like throat. The round-port full-bore pattern is the only one that passes a rigid pig.

Q: What is the difference between a plug valve and a ball valve?
A: Both are quarter-turn valves. A ball valve seals on two narrow seat rings, so it turns with less torque and suits clean service and fast automated isolation. A plug valve seals over a wide band around each port and has no open cavity for solids, so it handles dirty, sticky and abrasive fluids better, at the cost of higher torque.

Q: Which standards apply to plug valves?
A: API 599 covers metal plug valves from NPS 1/2 to 36 in steel, alloys and ductile iron, Class 150 to 2500 with ASME B16.34, but not 3-way or 4-way valves. API 6D covers pipeline plug valves for oil and gas. AWWA C517 and MSS SP-108 cover resilient-seated cast-iron eccentric plug valves for water and wastewater. Check the edition your project specification names.