Pressure Reducing Valves: Types, Sizing, Cavitation and Installation

Quick Answer

A pressure reducing valve (PRV) is a self-operated valve that holds a lower, steady downstream pressure whatever the inlet pressure or flow does. It is not a relief or safety valve: it can fail open, so where downstream equipment cannot take full inlet pressure, a relief valve still goes downstream of it. Direct-acting PRVs balance a spring against outlet pressure on a diaphragm or bellows; they are simple and fast, but outlet pressure droops as flow rises. Pilot-operated PRVs use a small pilot valve to drive the main valve, giving close control and more capacity per size; on water mains they are hydraulic control valves. Size a PRV on Kv at maximum and minimum flow, not line size: Kv = Q × √(SG ÷ Δp), and the right valve is often smaller than the pipe. Check cavitation with σ = (P2 − Pv) ÷ (P1 − P2), using absolute outlet pressure; one maker marks noise below about 0.8 and damage below about 0.5. Split high pressure ratios across two PRVs in series, and fit a strainer, isolation valves, gauges and a downstream relief valve.

A typical enquiry reads: "PRV, DN150, PN16, to match the pipe." The supply is 10 bar and the process line is rated for 6 bar. A line-size valve goes in, and within months it chatters at night, the downstream gauge creeps up when demand stops, and the pilot blocks with scale because there is no strainer. With no relief valve, a pilot fault that leaves the valve open will put full supply pressure into equipment never rated for it.

A PRV is a small control system in one body, chosen from flow and pressure data, not pipe size. This guide covers how PRVs work, how to size them, when cavitation calls for a second stage, and how to install them in water, steam and fire protection systems.

What a Pressure Reducing Valve Does, and What It Is Not

A PRV senses downstream pressure. When demand rises, outlet pressure dips and the valve opens further; when demand falls, it closes. At zero demand a good PRV closes tight and holds the outlet slightly above set point: lock-up. ("PRV" is also used for pressure relief valves; here it means pressure reducing valve.)

A torn diaphragm, a blocked pilot or debris under the seat can leave a PRV wide open. Where downstream equipment cannot withstand full inlet pressure, fit overpressure protection downstream: normally a relief or safety valve sized for the PRV's full-open flow at maximum inlet pressure (standard practice on steam) or, where that is impractical on large water stations, a second monitor regulator in series or a shut-off device; see our guide to pressure relief and safety valves.

Direct-Acting vs Pilot-Operated

Direct-acting

An adjustment spring pushes the plug open, outlet pressure acting on a diaphragm, bellows or piston pushes it closed, and the valve settles where the forces balance. To open wider, the spring must extend and its force falls, so outlet pressure falls as flow rises. This is droop, or proportional offset: the set pressure is met at one flow only. One regulator maker puts typical control of direct-operated designs at 10–20% at higher set pressures, against about ±1% for some designs at very low set pressures. Direct-acting valves are compact, cheap and fast, and suit small, steady, non-critical loads.

Pilot-operated

A small pilot senses outlet pressure and controls the pressure on the main valve's diaphragm or piston: the pilot senses, and line pressure does the work. The main valve's larger area amplifies small pilot movements, so droop is much smaller and swings in inlet pressure are largely balanced out. Capacity per size is higher: one maker notes that a DN50 (2 in) pilot-operated regulator can match a DN80 (3 in) direct-operated one. Pilot-operated valves suit large or variable loads and supplies where demand can fall to zero, but their small pilot passages make an upstream strainer essential.

Sensing element. Diaphragms are the most sensitive; metal bellows tolerate steam temperatures in small valves; pistons are compact and suit high pressure, but friction reduces their sensitivity and small ones can stick.

Water PRVs and Hydraulic Control Valves

On water mains and in large buildings, the PRV is usually a pilot-operated hydraulic control valve: a globe, angle or Y-pattern body with a diaphragm or piston actuator and a pilot circuit, powered by line pressure alone. Change the pilot and the same main valve becomes a sustaining, relief or flow control valve.

AWWA C530 covers these valves; its 2017 and 2022 editions are scoped for globe, angle and wye bodies, piston or diaphragm type, 37.5–1,500 mm (1½–60 in), water at 4–52 °C (40–125 °F), up to 2,070 kPa (300 psig) and a maximum line velocity of 4.6 m/s (15 ft/s). Check the current edition. For building valves, EN 1567 covers DN 8 to DN 100 for inlet pressures up to 16 bar, at up to 30 °C for cold water and 80 °C for hot.

Wide demand range. A town zone or a factory may draw a few per cent of its peak flow at night, and a large PRV throttling almost shut hunts and wears its seat. Add a small PRV in parallel at the target pressure and set the large valve slightly lower: the small valve carries low flows alone, and the large one opens as demand rises. Mains that need PRV stations also need air release valves at their high points.

Steam PRVs

Boilers run at high pressure because the steam is drier and the mains smaller; users need lower pressure for equipment ratings or a set saturation temperature. The PRV station needs:

• A separator and strainer upstream: wet steam wire-draws the seat, and pilots need clean, dry steam. Mount the strainer on its side so it does not fill with condensate
• Traps before the station and between the valves of a two-stage station, so condensate cannot collect and cause water hammer; see our guide to steam traps
• A larger outlet pipe: steam that is dry saturated at 10 bar g occupies 0.177 m³/kg; reduced to 2 bar g, it leaves slightly superheated and occupies about 0.646 m³/kg, about 3.6 times as much
• A safety valve downstream, sized for the PRV's full-open flow at maximum inlet pressure

Pressure reduction is throttling at constant enthalpy, so the steam leaves drier and often slightly superheated: dry saturated steam at 10 bar g (184 °C) reduced to 2 bar g leaves at about 159 °C, some 25 K above its 134 °C saturation temperature. One steam-valve maker recommends about 10:1 as a practical maximum ratio for a self-acting reducing valve (actuated control valves can go higher) and uses two stages beyond it, for example 25 to 5 bar g, then 5 to 1 bar g. For the boiler side, see industrial boilers and steam systems.

Sizing: Kv, Not Line Size

Kv is the flow of water in m³/h that passes the valve at a 1 bar pressure drop. Cv is the US equivalent, in US gpm at 1 psi, and Cv ≈ 1.156 × Kv. For liquids:

Kv = Q × √(SG ÷ Δp), with Q in m³/h, Δp in bar and SG the specific gravity (1.0 for water)

Steam and gas need compressible-flow equations or the maker's capacity tables. Size at two cases:

• Maximum flow at minimum inlet pressure (capacity): the valve should pass it partly open
• Minimum flow at maximum inlet pressure (stability and cavitation): a valve that must throttle almost shut will hunt, chatter and wire-draw its seat

A PRV chosen to match the pipe is usually oversized; our control valve guide explains the same logic, and PRV makers publish recommended flow ranges for each size.

Cavitation and When to Use Two Stages

As water accelerates through the seat, its pressure falls. If it reaches the vapour pressure, bubbles form, then collapse as pressure recovers downstream: noisy and, if sustained, destructive to seat, plug and body. Hydraulic control valve makers commonly use this cavitation index:

σ = (P2 − Pv) ÷ (P1 − P2)

P1 and P2 are inlet and outlet pressure; Pv is the vapour pressure of water, about 0.023 bar absolute at 20 °C. The numerator needs absolute outlet pressure: gauge pressure plus about 1.013 bar at sea level. Lower σ means worse cavitation. One maker's chart for its globe valves (below 40% open, normal water temperatures, low altitude) shows noise below about 0.8 and damage below about 0.5, acceptable only for short, infrequent periods. Makers using the ISA form, (P1 − Pv) ÷ (P1 − P2), get σ + 1, so their limits read one higher. Use the chart for the valve offered.

High inlet pressure, low outlet pressure and altitude all lower σ (at 1,000 m the atmosphere is only about 0.90 bar), and warm water raises Pv. The remedies:

• Two PRVs in series, each kept above its cavitation limit
• Anti-cavitation trim, which divides the drop into stages inside one valve
• A downstream orifice plate, which adds back-pressure; cheap, but suited only to a narrow flow range
• Stainless steel trim, which delays damage but does not prevent cavitation

Worked Example: A Factory Zone PRV

A factory takes water from a DN150 main at 10 bar g by day and 11 bar g at night. Its distribution runs at 4 bar g. Demand ranges from 120 m³/h at peak to 6 m³/h at night; the water is at 20 °C and the site is near sea level.

1. Capacity. Kv = 120 × √(1 ÷ 6) = 49 (Cv about 57).

2. Body size. At 120 m³/h the velocity is 6.6 m/s through a DN80 bore and 4.2 m/s through DN100 (nominal bores), against 1.9 m/s in the main. DN80 exceeds the 4.6 m/s scope limit of AWWA C530; DN100 does not. If the maker's table gives the DN100 valve a fully open Kv of 200, peak flow needs about 25% of it. The valve is two sizes smaller than the line.

3. Minimum flow. At night, Kv = 6 × √(1 ÷ 7) = 2.3, about 1% of the main valve's capacity: too close to the seat for stable control. Add a small parallel PRV sized on the night flow and set at 4.0 bar g, with the DN100 valve set slightly lower.

4. Cavitation. By day, σ = (4 + 1.013 − 0.023) ÷ (10 − 4) = 0.83. At night, σ = 4.99 ÷ 7 = 0.71 (0.70 at a site 1,000 m up): inside that maker's noise zone, above its damage zone. Check the small night valve on its own maker's chart.

5. A harder case. A gravity main delivers 16 bar g to a zone that needs 3 bar g. One stage gives σ = 3.99 ÷ 13 = 0.31, deep in the damage zone. Two valves in series, 16 to 7 bar g and 7 to 3 bar g, give 0.89 and 1.00, both clear of the noise zone; an equal-σ split would put the intermediate pressure at about 7.2 bar g.

Installation

A complete PRV station, in flow order:

• Isolation valves on both sides, so the PRV can be serviced
• A strainer upstream (see our guide to industrial strainers); on steam, a separator and trap as well
• Pressure gauges on both sides, for setting and fault-finding
• The PRV, with the straight pipe the maker asks for; connect any external sensing line to a straight run of outlet pipe, clear of the valve and fittings
• A relief or safety valve downstream, wherever the downstream system cannot take full inlet pressure
• A bypass, if supply must continue during maintenance; an open bypass passes full inlet pressure, so it relies on the same protection

Flush new pipework first: weld spatter and scale are what usually jam pilots and seats. At commissioning, check outlet pressure at no flow (lock-up) and at peak flow.

Fire Protection PRVs

Tall buildings and pumped fire systems often give the lower levels more pressure than sprinklers and hose outlets can take. Fire pumps are typically selected for the most demanding zone, and PRVs bring the other zones down; pump churn pressure is part of that check (see our guide to fire pumps). The NFPA requirements most often met:

• NFPA 13 (sprinklers): where system components are not listed for more than 12.1 bar (175 psi) and normal, non-fire pressure could exceed that, a listed PRV is set for an outlet pressure of no more than 11.4 bar (165 psi) at maximum inlet pressure. Gauges go on its inlet and outlet, and a relief valve of at least 15 mm (½ in) goes downstream, set no higher than 12.1 bar (175 psi)
• NFPA 14 (standpipes): pressures at hose connections are limited, broadly to 6.9 bar (100 psi) residual at 40 mm (1½ in) outlets for occupants or trained personnel and 12.1 bar (175 psi) at fire-service outlets; pressure-regulating hose valves or PRVs keep them there
• NFPA 25 (testing): periodic inspection and flow tests, including a full-flow test every five years. NFPA 13 also requires a means downstream of every sprinkler-system PRV for flow tests at system demand, so plan a test connection and a drain that can take that flow

Check which edition the authority having jurisdiction has adopted; insurers' data sheets can restrict PRV use further. See also sprinkler system valves and industrial fire protection systems.

Ordering Checklist

• Fluid and temperature: potable, raw or process water; saturated or superheated steam; air or gas
• Inlet pressure: minimum, normal and maximum, including static pressure at no flow
• Outlet pressure: set point and acceptable tolerance
• Flow: minimum, normal and peak
• Line and ends: size, pipe material, PN or ASME class; flanged, threaded or grooved; face-to-face length if replacing a valve
• Materials: body, trim, elastomers, and coating for buried or potable service
• Options: low-flow bypass, check feature, position indicator, solenoid or remote set-point
• Station items: strainer, separator and traps (steam), isolation valves, gauges, relief valve
• Fire service: listing or approval required, outlet range, code edition
• Documents: test and inspection records and material test certificates

Common Mistakes

1. Treating the PRV as overpressure protection. A torn diaphragm, debris under the seat or a blocked pilot supply can leave the valve wide open and send full inlet pressure downstream.

   Prevention: Wherever downstream equipment is rated below inlet pressure, fit overpressure protection: normally a relief or safety valve sized for the PRV's full-open flow, or a monitor regulator or shut-off device where full-flow relief is impractical.

2. Sizing to the pipe. An oversized valve throttles close to its seat, hunts and wire-draws.

   Prevention: Size on Kv at maximum and minimum flow; expect a valve smaller than the line.

3. One valve for a wide flow range. At night flows the valve chatters and outlet pressure swings.

   Prevention: Add a small parallel PRV at the target pressure, with the main valve set slightly lower.

4. One stage for a high pressure ratio. Noise first, then erosion of seat and body.

   Prevention: Check σ for every operating case; use two valves in series or anti-cavitation trim.

5. No strainer. Scale and weld debris jam pilots and seats.

   Prevention: Fit a strainer upstream and flush new pipework before installing the valve.

6. A sensing line in turbulent flow. The valve reads disturbed pressure and hunts.

   Prevention: Follow the maker's sensing-line location, on a straight run of outlet pipe.

7. Steam stations without separators, traps or a larger outlet pipe. Wet steam erodes the seat, condensate causes water hammer, and high outlet velocity brings noise.

   Prevention: Fit a separator, strainer and traps; size the outlet pipe for the expanded steam volume.

Supply from Kasko Makine

Kasko Makine supplies pressure reducing valves for water and fire protection service:

• 200X pressure reducing valve — a pilot-operated hydraulic control valve that uses the energy of the water itself, with no external power, to hold a constant, adjustable downstream pressure as inlet pressure and flow vary. PN10, PN16 and PN25 / Class 150, DN20 to DN800 (¾ to 32 in), pressure-tested to GB/T 13927 and inspected to API 598. For domestic, firefighting, industrial and municipal water supply systems
• Pressure reducing valve for fire protection systems — for zones where hydraulic calculations call for pressure adjustment. Threaded DN40–DN80 (1½–3 in), flanged DN40–DN250 (1½–10 in) and grooved DN40–DN200 (1½–8 in); inlet pressure 12.1 or 20.7 bar (175 or 300 psi), with outlet adjustable from 2.1 to 11.4 bar (30–165 psi); maximum working temperature 82 °C. Body and cover in ASTM A536 ductile (nodular) iron, internal parts in ASTM B61 bronze, pilot system in ASTM B62 bronze with type 303 stainless steel parts, and an elastomer diaphragm. The product page lists UL and ULC listing

Engineering support: Send the fluid, inlet pressure range, required outlet pressure and flow range. We will check Kv, body size and cavitation index against your data, and propose the valve and the station components around it.

Send your pressures, flow range and line size for a PRV sizing proposal and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Control Valves and Actuators: Types, Cv Sizing and Selection Guide
• Pressure Relief and Safety Valves: API 520/526 Types, Sizing and Selection
• Air Release Valves for Water Pipelines: Types, Placement and Sizing
• Steam Traps: Types, Selection and Why 20% of Them Are Failed Right Now
• Industrial Strainers and Filters: Y, Basket, Duplex Types and Selection
• Industrial Fire Protection Systems: Sprinklers, Foam, Gas and Hydrants

Frequently Asked Questions

Q: What is a pressure reducing valve?
A: A pressure reducing valve is a self-operated valve that holds a lower, steady downstream pressure, whatever the inlet pressure or flow does. It senses outlet pressure and throttles to keep it at set point, closing tight at zero flow. Direct-acting types balance a spring against outlet pressure; pilot-operated types use a small pilot valve to drive a larger main valve for closer control.

Q: What is the difference between a pressure reducing valve and a pressure relief valve?
A: A pressure reducing valve senses downstream pressure and throttles continuously to hold a lower outlet pressure. A pressure relief or safety valve senses system pressure, stays closed in normal operation and opens only to discharge fluid at its set pressure. A reducing valve can fail open, so where downstream equipment cannot take full inlet pressure, a relief valve is fitted downstream of it.

Q: Should I choose a direct-acting or a pilot-operated pressure reducing valve?
A: Choose direct-acting for small, steady, non-critical loads: it is compact, cheap and fast, but its outlet pressure droops as flow rises. Choose pilot-operated for larger or variable loads, for supplies where demand can fall to zero, and for close pressure control. Pilot-operated valves carry more flow per size, but their small pilot passages need clean fluid and an upstream strainer.

Q: How do you size a pressure reducing valve?
A: Size it on the flow coefficient, not the pipe size. For water, Kv equals the flow in cubic metres per hour multiplied by the square root of specific gravity divided by the pressure drop in bar, and Cv is about 1.156 times Kv. Check maximum flow at minimum inlet pressure for capacity, and minimum flow at maximum inlet pressure for stability and cavitation.

Q: How do you prevent cavitation in a water pressure reducing valve?
A: Check the cavitation index: absolute outlet pressure minus vapour pressure, divided by the pressure drop. One maker's chart shows noise below about 0.8 and damage below about 0.5, but limits vary by valve design. If the index is too low, use two valves in series, anti-cavitation trim, or a downstream orifice plate for a narrow flow range. Altitude and warm water both lower the index.

Q: What is a low-flow bypass on a pressure reducing station?
A: It is a small pressure reducing valve installed in parallel with the main valve. At very low demand, such as night flow, a large valve would throttle almost shut and hunt or chatter. The small valve is set at the target pressure and the main valve slightly lower, so the small valve carries low flows alone and the main valve opens as demand rises.

Q: What does NFPA 13 require for pressure reducing valves in sprinkler systems?
A: Where components are not listed for more than 12.1 bar (175 psi) and normal pressure could exceed that, NFPA 13 requires a listed pressure reducing valve set for an outlet of no more than 11.4 bar (165 psi) at maximum inlet pressure, with gauges on its inlet and outlet and a relief valve of at least 15 mm downstream. NFPA 13 also requires a downstream means for flow testing at system demand; NFPA 25 adds periodic and five-yearly full-flow tests.