Pressure Relief Valves and Control Valves: What They Do and How to Specify Them

Isolation valves are bought by size and class. Pressure relief valves and control valves are bought by performance, and a requisition that lists only line size and flange rating will come back with a dozen questions or, worse, the wrong valve. This article explains what each of these two valve types does, where the common mistakes are made, and which process data the buyer must hand to the supplier so that the valve can be sized and selected correctly.

Two different jobs

A pressure relief valve (PRV) is a safety device. It stays closed in normal operation, opens automatically when system pressure reaches its set pressure, discharges enough fluid to keep the protected equipment within its limit, then recloses. It is the last line of defence and is governed by pressure-equipment codes.

A control valve is a process device. An actuator positions it continuously, under command from a controller, to regulate flow, pressure, level or temperature.

Pressure relief valve terminology

Set pressure is the inlet pressure at which the valve begins to open. Overpressure is the rise above set pressure needed to reach rated capacity, typically 3 to 10 percent for compressible fluids and 10 to 25 percent for liquids. Blowdown is the difference between set and reseat pressure, usually under 10 percent for gases and up to 20 percent for liquids. Backpressure is the outlet pressure, made up of the superimposed pressure already in the discharge system and the built-up pressure created by the valve's own discharge.

In US usage a safety valve pops open and suits gases and steam, a relief valve lifts progressively and suits liquids, and a safety relief valve does either depending on the fluid; the generic term is pressure relief valve. European practice calls all of these safety valves under EN ISO 4126.

Choosing the relief valve design

A conventional spring-loaded valve has its spring housing vented to the outlet, so backpressure acts on the disc and shifts the opening pressure. It is the economical choice for discharge to atmosphere or a low-pressure header and often the best choice for general steam duty, but it is unsuitable where variable backpressure exceeds about 10 to 15 percent of set pressure.

A balanced bellows valve adds a bellows whose effective area equals the seat area, so backpressure has little effect on set pressure. It costs more and is used on process relief headers in oil, gas and petrochemical plants and wherever precise set pressure and blowdown are needed; the bellows also isolates the spring from corrosive fluid. A pilot-operated valve uses a small pilot to control a piston main valve; it holds tight close to set pressure, achieves blowdown of around 2 percent and is preferred for large capacities, but its pilot tubing can block with debris or condensate, so it suits clean services.

Set pressure must never exceed the maximum allowable working pressure of the protected equipment, and the valve must reach full capacity within the overpressure the code allows: usually 3 to 5 percent for steam boilers, 10 percent for most vessels and up to 20 percent for fire cases under API 520. Refer to ASME Section VIII and API 520 for the accumulation limits that apply. Set pressure is normally adjusted to a tolerance of about 3 percent and the valve sealed afterwards.

Installation decides whether the valve works. Non-recoverable pressure loss in the inlet line must not exceed 3 percent of set pressure, otherwise the valve can chatter, which destroys seats and can fracture inlet piping. The outlet must keep built-up backpressure within the limits of the valve type. Flanged steel valves are normally bought to API 526, which standardises inlet and outlet sizes, flange classes and orifice letters D through T; effective orifice areas run from 71 mm2 (D) to 16,774 mm2 (T), and the sizing equations in API 520 Part I select the letter.

What the supplier needs to size a relief valve

Give the fluid and its state (gas, vapour, liquid, two-phase), molecular weight or density, relieving temperature, the governing relief case and its required capacity, set pressure, allowable overpressure, superimposed and built-up backpressure, connection sizes and classes, body and trim materials, and whether a bellows, open or closed bonnet or lifting lever is required. The relief case (blocked outlet, fire, control valve failure, thermal expansion) must come from the plant designer, because capacity depends on it.

Control valves: sizing by flow coefficient

Cv is the flow of water in US gallons per minute at 60 F through the valve with a pressure drop of 1 psi; Kv is the metric equivalent in cubic metres per hour at 1 bar. The required coefficient is calculated for maximum, normal and minimum flow with the pressure drop available at each, and a valve is chosen whose range covers all three. Oversizing is the most common error: a valve controlling at 10 percent open hunts, erodes its seat and loses accuracy. Control valves work best between roughly 20 and 80 percent of travel.

The trim sets the inherent characteristic, the relationship between lift and flow at constant pressure drop. Linear trim passes flow in proportion to lift and suits duties where the pressure drop across the valve stays nearly constant, such as three-port valves in balanced water circuits. Equal percentage trim increases flow by a constant percentage of the previous flow for each increment of lift; it tolerates oversizing, copes with pressure drops that fall as flow rises, and is the usual choice for steam, compressible fluids and most two-port duties. Fast-opening trim gives most of the flow early in the travel and is used for on-off duty. The installed characteristic differs from the inherent one because the available pressure drop changes with flow, which is why equal percentage trim often behaves almost linearly in a real system.

Specifying the control valve package

Beyond Cv and characteristic, settle the body type (globe for most modulating duties, rotary ball or butterfly for high capacity or dirty fluids), body material and pressure class to ASME B16.34, end connections, actuator type and fail action (open, closed or last position) on loss of air or power, positioner and signal, and the permitted seat leakage.

Seat leakage is defined by ANSI/FCI 70-2 in six classes. Class II allows 0.5 percent of rated capacity, Class III 0.1 percent and Class IV 0.01 percent, tested with air or water at a modest differential; Class IV is the usual metal-seated standard. Class V is a water test at full service pressure, expressed per inch of port diameter per psi of differential, and is specified for critical shut-off. Class VI applies to soft-seated valves and is measured in bubbles per minute with air or nitrogen; refer to the standard for the table by port size. A control valve is not a tight-shut-off isolation valve; if both functions are needed, add a separate isolation valve.

State the noise and cavitation conditions as well. High pressure drops on liquids cause cavitation that destroys trim within months; high drops on gas and steam cause excessive noise. Both need special trim and must be known at quotation stage.

Key takeaways

• A relief valve protects equipment under ASME VIII, API 520, API 526 and EN ISO 4126; a control valve regulates a process and is never a substitute for one.
• Specify a relief valve by relieving capacity, set pressure, overpressure, backpressure and fluid state; pick conventional, balanced bellows or pilot-operated according to backpressure and service.
• Keep inlet pressure loss below 3 percent of set pressure.
• Size control valves by Cv or Kv across minimum, normal and maximum flow and aim to operate between 20 and 80 percent of travel.
• Choose equal percentage trim for most two-port and steam duties, linear trim where pressure drop is constant.
• Specify seat leakage to ANSI/FCI 70-2 (Class IV metal seat, Class VI soft seat) and state the fail action, actuator and signal type.

Frequently asked questions

Can a control valve be used as a pressure relief valve?

No. A control valve depends on an external controller, actuator and power supply, any of which can fail, and carries no code certification as a relief device. Overpressure protection must come from a self-acting relief valve or rupture disc sized and set to the applicable code.

Why is my relief valve chattering?

The usual causes are inlet pressure loss above 3 percent of set pressure, excessive built-up backpressure on a conventional valve, or a valve heavily oversized for the actual relieving flow.

What information do I need for a budget price on a control valve?

Fluid, inlet pressure, pressure drop and flow at maximum, normal and minimum conditions, temperature, line size, pressure class and material, actuator type and fail action, and the required leakage class. Without the flow data the valve cannot be sized.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies pressure relief valves and control valves within its industrial valves range for plant and building services projects. Send us the process data sheet or the information listed above and our team will coordinate sizing and selection with the manufacturer before quoting; contact us to begin.