S1. General

S1.1 Introduction

A Relief safety valve is a safety device and in many cases the last line of defence. It is important to ensure thatthe relief safety valve is well designed and capable to operate at all times and under all circumstances. A safety felief valve is not a process valve or pressure regulator and should not be misused as such. It should have to operate for onepurpose only: overpressure protection.

S1.1 Scope

This standard has been developed to standardize the method of safety relief valve calculations in accordance with API, ASME codes and local codes.

S1.2 Calculation Sheet

For the calculation of a safety relief valve, the following standard calculation forms can be used:

• Safety relief valve calculation
• Attachment “A” (External fire)
• Attachment “B” (Heat exchangers)
• Attachment “C” (Miscellaneous)

The main safety relief valve calculation sheet, is established to evaluate the governing hazard and to calculate the area of the relief valve andto evaluate the quantity which has to be relieved by the safety relief valve.

S1.3 References

1. API Recommended Practice for the design and installation of pressure relieving system in refineries, API RP 520, second edition, Sept. 1960.
2. The design and construction of pressure relieving systems by Nels E. Sylvander and Donald L. Katz, April 1948.
3. Data Handbook on Hydrocarbons by J.B. Maxwell, 5th edition.
4. Engineering Data Book by the National Gasoline Supply Men’s Association (NGSMA), 7th edition 1957.
5. Chemical Engineers’ Handbook, John H. Perry, 3rd edition, 1950.

S1.4 Definitions

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

Relief valve
An automatic pressure relieving device actuated by the static pressure upstream of the valve, which opens in proportion to the increase in pressure over the opening pressure. It is used primarily for liquid service.

Note: This type is normally only used in the size 3/4” x 1” or 1” x 1” for thermal expansion.

Safety relief valve
An automatic pressure actuated relieving device suitable for use as either a safety or relief valve, depending on application.

Note: This type is used for general applications.

Set pressure

1. Liquid service:
   for a relief or safety relief valve on liquid service, the set pressure, in pounds per square inch gauge, is to be considered the inlet pressure at which the valve starts to discharge when discharging against atmospheric conditions.
2. Gas, vapor or steam service:
   for a safety relief valve on gas, vapor or steam service, the set pressure, in pounds per square inch gauge, is to be considered the inlet pressure at which the valve pops when discharging against atmospheric conditions.

Popping pressure
Same definition as set pressure for gas, vapor or steam service, when discharging against back pressure. (Pressure at which the valve pops when in use.)

Differential test pressure
The pressure differential in pounds per square inch between the set pressure and the constant superimposed back pressure.

It is applicable only when a conventional type safety relief valve is being used in service against constant superimposed back pressure.

Net spring setting
Same definition as for differential test pressure.

Cold differential test pressure
The set pressure or the differential test pressure at which a safety relief valve must be set on cold fluid on a test drum plus a predetermined increase in the cold fluid setting, in pounds per square inch, that will result in the valve opening at the correct set pressure in service when the actual service temperature is higher than that of the cold fluid which is used in the test drum. This is the pressure at which the valve should be set on repair shop testing facilities.

Note:The difference in cold and hot setting is the result of change in characteristic of the spring.

Cold net spring setting
Same definition as for cold differential test pressure.

Operating pressure
The opening pressure of a vessel is the pressure in pounds per square inch gauge, to which the vessel is usually subject in service. A vessel is usually designed for a maximum allowable working pressure, in pounds per square inch gauge, which will provide a suitable margin above the operating pressure in order to prevent any undesirable operation of the relief device. (It is suggested that on processing vessels this margin be approximately 10 percent, or 25 psi - whichever is greater.)

Note:

1. Take care for reciprocating engines, where peak pressure might reach the set pressure of the valve.
2. When setting closer to the operating pressure is required, pilot operated safety valves or “0” ring seal might be considered.

Design pressure
The gauge pressure at which the vessel has been designed.

Note: Safety relief valves are usually set at this pressure.

Maximum allowable working pressure
The maximum gauge pressure, at the coincidental design temperature, permissible at the top of a vessel in its operating position, and which could be the basis for the upper limit in pressure setting of the safety relieving devices for any specific operation.

Note:

1. When designing a vessel, the calculations will result in a material thickness. This thickness is rounded off to the first higher thickness which is commercially available. Recalculation of the vessel using actual plate thickness will result in a maximum allowable working pressure which is usually higher than the design pressure.
2. Usually recalculation is not done (difference in plate thickness is low for European standards), in that case maximum allowable working pressure is equal to the design pressure.

Overpressure
Pressure increase over the set pressure of the primary relieving device is overpressure. It is the same as accumulation only when the relieving device is set at the maximum allowable working pressure of the vessel.

Accumulation
Pressure increase over the maximum allowable working pressure of the vessel during discharge through the pressure relief valve, expressed as a percentage of that pressure, or pounds per square inch, is called accumulation.

Notes:

1. For European plants, it is usual to take an accumulation of 10% . For American plants a value of 20% may be taken for external fire. 25% for liquid relief and in all other cases 10%.

Blowdown
Blowdown is the difference between the set pressure and the resealing pressure of a pressure relief valve, expressed as percentage of the set pressure, or pounds per square inch.

Note: A blowdown between 4% and 5% is usually specified. Care is to be taken that the inlet piping does not have a total pressure loss in excess of 3%. The total pressure loss shall include the velocity head loss ( 1/2 rV2). Note however, that the friction loss should not exceed 1% and the velocity head loss should not exceed 2% of the allowable pressure for capacity relief.

Lift
The rise of the valve disc in pressure relief valve when the valve disc moves from the closed position is called lift.

Back pressure
Pressure on the discharge side of a pressure relief valve.

1. Constant back pressure:
   back pressure which does not change appreciably under any condition of operation whether or not the pressure relief valve is closed or open.
2. Variable back pressure:
   back pressure which develops as a result of the conditions outlined below.
   a) Built-up back pressure: variable back pressure as a result of flow through discharge piping after a single pressure relief valve starts to open.
   b) Superimposed back pressure: variable back pressure that is present before the pressure relief valve starts to open.

Note: Variable back pressure on conventional safety relief valves shall never exceed 10% of the gauge set pressure.

Variable back pressure on bellow seal safety relief valves shall never exceed 50% of the absolute set pressure as that is the limit for critical flow, on which the formulae are based.

Coefficient of discharge
Correction factor for the nozzle area, due to contraction etc., include all deviations from ideal flow through the nozzle.

Note:
For full lift safety relief valves, this factor is usually 0.97.
For relief valves at 25% accumulation: 0.64.
For relief valves at 25% accumulation: 0.64.
These low coefficients correct also for the reduction in area which occurs at lower lifts of the disc above the seat.

General rules for the use of safety relief valves
Following, a few general rules will be given for the application of safety relief valves. These rules are only to be used as a guidance.

Safety relief valves are required:

1. On all vessels containing fluids which may vaporize when the vessel is subject to fire.
2. On all vessels where a closed outlet valve may cause an over pressure.
3. On all reciprocating pumps and compressors, if not protected internally.
4. On the outlet of turbines, where the design pressure of the low stage casing is lower than the inlet pressure.
5. On all distillation and fractionating towers where reflux or cooling water failure may cause excessive vaporization.
6. On heaters and furnaces where low flow may cause cooking.
7. On heat exchangers or heaters where the cold medium may be blocked.
8. On all storage tanks, not vented to atmosphere, where vapor pressure or pumping may cause over or under pressure. In this case special combined pressure vacuum relief valves are to be specified. (Breather valves).
9. In cases of long lines, containing liquid, which may be blocked when exposed to the sun (for example butane, propane).
10. In all cases where a volatile liquid may get into a vessel under conditions that it vaporizes where the vessel is not designed for the vapor pressure.
11. In cases where non-condensable accumulate in a condenser, decreasing the efficiency, so that a pressure rise may occur.
12. Tube failure in heat exchangers, which may cause vaporization, or excessive pressure on the shell.
13. Control valve failure or instrument failure which either results in one of the hazards 1 through 12, or in any other hazard as for example a reducing station, failing in the open position.

Notes:

1. Where more vessels are connected, so that they cannot be separated by block valves, they are to be considered as one system. (Except when local codes imply separate valves.)
2. In cases where equipment has to be protected against two or more of the above mentioned hazards, the safety valves shall be calculated for the condition causing the highest flow rate to be relieved, thus assuming that only one hazard will occur at the same time. (Single risk concept.)
3. It is evident that when a hazard is the result of another hazard, both hazards are to be taken into account.

SPRING LOADED SAFETY VALVES

The spring force Fs is transmitted via the spindle to the plate closing against the nozzle, which together with the plate closes the process and ensures sealing. This condition is maintained as long as the
spring force is greater than the force Fp generated by the pressure at the valve inlet.

PILOT-OPERATED SAFETY VALVES

Pilot-operated safety valves limit the pressure in the system by compensating for the overpressure by opening the main valve. They are self-medium controlled and, as with all medium-loaded safety
valves, the same pressures act on both sides of the sealing disc. However, a greater force acts in the closing direction due to surfaces of different sizes.

S2. Formulae

Belowa summary of formulae has been given. Most of these formulae already appear on the calculation sheet, but this summary also gives some other formulae which are to be used in special cases.

The formulae given on the calculation sheet for evaluation of nozzle area for vapors is a safe approximation: when accurate calculations are to be made, formula number 2 has to be used in which the “C” factor can be evaluated from formula 2A, or from Binder IV with curves and tables.

Formula 4 gives the nozzle are for steam when the safety valve has to be calculated as per ASME boiler code. Formula 10 gives a quantity of thermal expansion when no normal size is taken. See 68, liquid expansion.
In each formula for vapors the super compressibility factor (Z) can be included as a multiplier of the absolute temperature (also under the square root sign).

This will result in a smaller valve size.
Leaving it out always gives a safe approximation.

UNITS OF MEASUREMENT

SUMMARY OF FORMULAE