SAFETY RELIEFVALVE SIZING

BASICS OF RELIEF VALVES

Types of Relieving Devices

Pressure Relief Valve - A pressure relief device designed tore-close and prevent the further flow of fluid after normalconditions have been restored.

Safety Valve - An automatic pressure relieving deviceactuated by the static pressure upstream of the valve,and characterized by rapid full opening or pop action. It is used for steam, gas or vapor service

Relief Valve - An automatic pressure relieving deviceactuated by the static pressure upstream of the valve,opening in direct proportion to the pressure increase. It is used primarily for liquid service.

Safety Relief Valve - An automatic pressure relievingdevice suitable for use as either a safety or relief valve,depending on application.

Conventional Safety Relief Valve - A safety relief valvehaving its spring housing vented to the discharge sideand which is directly affected by fluctuations inbackpressure.

Balanced-Bellows Safety Relief Valve - A safety reliefvalve incorporation in its design a means of compensationfor fluctuations due to backpressure.

Pilot Operated Pressure Relief Valve - A pressurerelief valve in which the major relieving valve is combinedwith and is controlled by a self-actuated auxiliarypressure relief valve.

Temperature Relief Valve (P&T) - A pressure relief valvewhich may be actuated by external or internal temperature orby pressure o the inlet side.

Rupture Disc - A nonre-closing pressure relief device actu-ated by inlet static pressure and designed to function by thebursting of a pressure containing disc.

Breaking Pin Device - A nonre-closing pressure relief deviceactuated by inlet static pressure and designed to function bythe breakage of a load-carrying section of a pin whichsupports a pressure containing member.

Parts

Nozzle - The pressure containing element which constitutesthe inlet flow passage and includes the fixed portion of theseat closure. The nozzle can be of two designs: Full-nozzle orSemi-nozzle.

Full Nozzle - a single member extending from the faceof the inlet flange to the valve seat.

Semi-Nozzle - the lower part of the inlet throat isformed by the body casting and the upper part is valveseat threaded or welded into the valve body

Disc - The pressure containing movable element of a pressurerelief valve which effects closure.

Trim - Internal parts. Specifically the seat (nozzle) & disc.

Orifice - A computed area of flow for use in flow formulas todetermine the capacity of a pressure relief valve.

Huddling Chamber - The annular pressure chamber locatedbeyond the valve seat for the purpose of generating a poppingcharacteristic.

Lifting Device (Lever) - A device to manually open a pres-sure relief valve by the application of external force to lessenthe spring loading which holds the valve closed. Lifting devicescan be an open lever or a packed lever (fully enclosed design).

Balanced Bellows - A bellows designed so that the effectivearea of the bellow is equivalent to that of the valve seat, there-by canceling out the additive effect of the backpressure. Bal-anced bellows are used for the following reasons: Backpres-sure is excessive or variable, fluid is highly viscous or slurry, orthe fluid is corrosive to the upper works of the valve.

BASICS OF RELIEF VALVES, CONTD.Operational CharacteristicsRated Capacity - The measured flow at an authorized per-cent overpressure permitted by the applicable code. Ratedcapacity is generally expressed in lbs/hr for Steam, SCFM forgases and GPM for liquids.

Operating Pressure - The pressure to which the vessel isusually subjected in normal service.

Set Pressure - The inlet pressure at which the valve isadjusted to open under service conditions. On a relief valve(liquid service), this is considered the point at which the firstcontinuous stream of water starts to discharge and runsvertically down from the outlet of the valve. On a safety valve(Steam, air or gas) it is the inlet pressure at which the valvepops (not the point of first audible simmer or warning).

Cold Differential Test Pressure (CDTP) - The inlet staticpressure at which a pressure relief valve is adjusted to openon the test stand. This test pressure includes corrections forservice conditions of backpressure and/or temperature.

Simmer (Warn/Pre-Open) - The audible or visible escape of fluid between the seat and disc at an inlet static pressurebelow set pressure and at no measurable capacity. All pres-sure relief valves will have some simmer. Typically a metalseated valve will simmer at 90% of the set pressure and asoft seated valve will simmer at 95% of the set pressure.

Coefficient of Discharge - The ratio of the measuredrelieving capacity to the theoretical relieving capacity.

Maximum Allowable Working Pressure (MAWP) - Themaximum gauge pressure permissible in a vessel at a desig-nated temperature.

Overpressure - The pressure increase over the set pressureof a pressure relief valve, usually expressed as a percentageof the set pressure.

Accumulation - The pressure increase over the maximumallowable working pressure (MAWP) of the vessel during dis-charge through the pressure relief valve usually expressed asa percentage of the set pressure.

Blowdown - The difference between the actual set pressureof a pressure relief valve and the actual reseating pressure,expressed as a percentage of the set pressure or in pres-sure units.

Leak Test Pressure - The specified inlet static pressure atwhich a standard quantitative seat leakage test is performed.

Backpressure - Pressure from the discharge side of apressure relief valve. There are two types of backpressure:Constant Backpressure and Variable Backpressure.

Constant Backpressure - Backpressure which doesnot change appreciably under any condition of operation,whether or not the pressure relief valve is open or closed.A conventional relief valve may be used, provided thespring setting is reduced by the amount of the constantbackpressure.

Variable Backpressure - Pressure from the outlet sideof the relief valve as a result of the Superimposed VariableBackpressure or Built-up Backpressure.

Superimposed Variable Backpressure - A variablebackpressure that is present before the pressure reliefvalve starts to open. It is usually the result of one or morevalves discharging into a common header, causing avarying degree of backpressure on each valve connectedto the system. It will cause the spring set pressure to beincreased an amount equal to the backpressure. It canbe negated by the use of a bellows-style relief valve.

Built-up Backpressure - Pressure which develops atthe valve outlet as a result of flow after the pressure reliefvalve has opened.

Lift - The actual travel of the disc away from the closedposition when a valve is relieving.

Chatter - Abnormal, rapid reciprocation movement of thedisc on the seat of a pressure relief valve.

Safety Valves are pressure relief devices actuated by inletstatic pressure and characterized by rapid opening orpop action. The difference between Safety Valves fromdifferent manufacturers is how well they do this.

Spence Figure 31 Safety Valves Dual Ring Control allowsfor finer adjustment of the popping action and length ofblowdown. This allows exceptional flow efficiency andmaximum lifting force while minimizing system energy loss.

DUAL RING CONTROL

CLOSED OPENING OPENSystem pressure is pushing up-ward against the disk which is heldclosed by the downward force ofthe spring against the spindle.

When system pressure rises above the setpressure of the spring, the disc begins to lift.This simmer/warn stage allows system pressureto enter the huddling chamber where it actson a larger, secondary area of the disc. Thismagnified force causes the valve to pop open.

As pressure increases, the disccontinues to lift until fully open.When pressure is reduced to alevel below the set point of thevalve, the spring force against thespindle will snap shut the disc.

GENERAL1. Recommend a 20% or 10 PSIG differential between oper-

ating and set pressure, whichever is greater. The set pres-sure of each pressure relief valve must be in conformancewith limits specified in the appropriate ASME code.

2. Relieving Capacity

a. ASME Section I - The minimum required relieving capacityof the pressure relief valve for all types of boilers shall notbe less than the maximum designed steaming capacityas determined by the Manufacturer and shall be basedon the capacity of all the fuel burning equipment aslimited by other boiler functions. (ASME Section I, PG-67.2.1, 1998)

b. ASME Section VIII - The minimum required relieving capa-city shall be sufficient to carry off the maximum quantitythat can be generated or supplied to the attachedequipment without permitting a rise in pressure within thevessel with appropriate overpressure condition above themaximum allowable working pressure.

3. Pressure relief valves should not be oversized. Oversizing apressure relief valve will cause chatter. A multiple valveselection should be used in order to eliminate the possibilityof chattering. Use a multiple valve installation when:

a. The maximum specified capacity requires selection of apressure relief valve greater than 6 inch pipe size.

b. When it is more economical to install two smaller valvesthan one very large one.

c. If the normal operating capacity of the system is lessthan approximately 50% of the valve capacity. In thiscase the volume is not sufficient to keep the valve in itsopen position and the spring will push the valve closedcausing chattering. The first pressure relief valve shouldbe sized on the normal operating capacity and theremaining s