Practical Process Engineering

Balanced safety valve: As disscussed earlier, superimposed bp affects the conventional safety valves and system pressure under the disk must overcome on both the spring force and the force resulted from the superimposed bp. The balanced safety valves are equipped with a bellows or piston which covers the area of the disk and superimposed bp no longer can affect the safety valve. Hence, safety valve only needs to conquer the spring force to lift. Following pictures show how bellows or piston cover the disc area so that superimposed bp con not affect the disc lift.

Relation between set pressure, relieving prssure and closing pressure. The difference between set pressure and relieving pressure is overpressure and the difference between set pressure and closing pressure is so called blowdown. When the disc lifts, pressure is constant and after a while it starts increasing toward relieving pressure.

Spring loaded conventional safety valve with threaded connecction- API 520

Spring loaded conventional safety valve-API 520

Spring loaded conventional safety valve

While there are still many more important terms in the safety valve worlds, we stop this part here and postpone the rest to later. Here is an introduction to different types of safety valves: Three major safety valves are: 1- Conventional 2- Ballanced (Bellows or Piston) 3- Pilot operated Conventional safety valve: It is spring loaded. The spring force keeps the disk to be on the seat and the valve remains closed untill static pressure under the seat reaches set pressure and there is enough force to push the disk up. When the disk lifts up, spring contracts snd fluid flows out. There is an allowable overpressure (say 10%) causes system pressure goes up. When safety valves reaches full open position, the system pressure reaches relieving pressure. Compare to other types of safety valves, conventional is cheaper. However, the bp can affect its performance. Actually, the static pressure under the disk to lift it up must be enough not only to overcome the spring force, but also to conquer the superimposed bp. Also, built-up bp can affect its performance. In short, this type of safety valve is only suitable for low total built-up bp system. API-520 explicitly mentions: Max built-up bp <= Max allowable overpressure It means if set pressure= MAWP= 100 psig and allowable overpressure is 10%, the built-up bp can be up to 10 psi. Or if this safety valve is a sinlge one a vessel for fire case with 21% allwoable overpressure, built-up bp can be as high as 18% or even 21%. For multiple safety valves, the same rule controls built-up bp; If the first safety valve sets at MAWP and second one set at 1.05 * MAWP, built-up bp can be as high as 10% of MAWP and 16% of MAWP for the first and second safety valves respectively. Please note: 1- The bp criterion for conventional safety valve is based on built-up bp nor superimposed bp neither total bp. 2- Conventional safety valve application is not limited to 10% bp (as many people think or you may read it in many references). The type of bp should be clearly stated and, also, API limits built-up bp to maximium allowable bp. 3- If set pressure is less than MAWP, overpressure can be higher than 10%, 16% or 21% (limited to maximum allowable accumulation). Therefore, built-up bp can be higher than 10%, 16% or 21% overpressures (limited to maximum allowable accumulation). 4- Conventional safety valves can be employed for bp even higher than 50% with some specific adjustments. This option is not always available and you can ignore it. 5- Here we only focus on spring loaded safety valves. There is weight loaded conventional safety valve which is out of scope of this discussion and we talk about it in future. Let's take a close look at conventional safety valve structure.

two more terms: Closing pressure: When static pressure under a safety valve disk reaches the set pressure, it pops and there is always an allowable overpressure (10%, 16% or even 21%) and system pressure reaches relieving pressure ultimately. However, when system pressure comes down, safety valve closes at a pressure less than the set pressure and wa call it "Closing Pressure" that is adjustable. Blowdown: Difference between set pressure and closing pressure. It is adjustable. It is expressed in terms of pressure or even some % of the set pressure.

Okkkkkkkkkkk! There are atill more terms you have to be familiar before successful sizing of a safety valve. Backpressure: The pressure right after a safety valve outlet nozzle is so called backpressure (bp). Regarding the safety valves, three kinds of bp's are defined: 1- Superimposed bp: The bp when the valve is closed. Two types of superimposed bp's are defined: a) Constant superimposed bp: When the safety valve is connected to a fixed-pressure environment like atmosphere. Atmospheric safety valves are mainly installed on non-toxic services such as N2, O2, CO2, Steam and Air. b) Variable superimposed bp: When the safety valve discharges to a variable-pressure environment like a relief header (flare header) connected to several other safety valves. Such safety valve is connected to that relief header through a tail pipe. Other safety valves can discharge to this common header and the superimposed bp can not be a constant number. 2- Built-up bp: Once the safety valve opens and fluid starts flowing, a specific bp resulted from the fluid flow appears right after the outlet nozzle. It is built-up bp resulted from fluid flow. 3- Total bp: Superimposed bp + built-up bp= Total bp Bp's are expressed in pressure or ratio of outlet pressure to relieving pressure (ratio of absolute pressures).

Note: We can use single or multiple safety valves to support fire and non-fire contingencies providing that the accumulation limits apprpriately to 110% and 116% for single and multiple valves installation respectively. It means a single valve that is sized to cover both nonfire and fire contingencies can't adopt 121% as the maximum allowable accumulation and its accuumulation is limited to 110% MAWP. Also, multiple valve instalation can no longer sized for 121% accumulation and their accumulation is limited to 116%

The first on e sets at MAWP and the second one can have a set pressure not more than 105% MAWP. Both have the same ultimate relieving pressure. But the first one with less set pressure has greater overpressure.

Please note: Maximum set pressure for single safety valve for fire case = MAWP Maximum allowable accumulation pressure for single safety valve= 121% MAWP Let's take a look at a multiple safety valve installation example for fire case:

This example from API-520 shows how would be the accumulation if a single safety valve protects a vessel against external fire. The set pressure can be =< MAWP. If set pressure= MAWP, accumulation= overpressure. If set pressure< MAWP, overpressure will be more than the previous one when set pressure= MAWP, but it is still limited to maximum allowable overpressure. In this example, both safety valves have the same accumulation and relieving pressure ultimately, but with different overpressure. ASME limits maximum allowable accumulation for fire contingency to 121% MAWP.

An example from API-520: mutiple safety valve installation for nonfire contingency. The first valve is set at MAWP and the second one is set at 105% MAWP. Please note that maximum allowable overpressure of multiple safety valve installation for nonfire contingencies is 116% MAWP. It means the first valve makes acccumulation up to 116% of its set pressure (normally 110% or 116% are chosen) and the second one set at 105% MAWP makes accumulation up to 10% of its set pressure (normally 110% is chosen).

An example from API-520 for single safety valve in nonfire contingency. Please note that how overpressure varries when set pressure is not the same as MAWP. While both examples have the same MAWP and the same accumulation limit (110% of MAWP) and the same relieving pressure, the second one has greater overpressure because its det pressure is less than MAWP. Maximum overpressure of a single safety valve for nonfire contingency= maximum allowable accumulation= 110% MAWP

For single valve for nonfire contingencies, Max set pressure= MAWP. But for multiple valves, the first one sets at MAWP and the second opens at 1.05 * MAWP. ASME allows us to have maximum 1.21 * MAWP for accumulation in fire contingency. If first safety valve is installed at MAWP for nonfire antingency with 10% overpressure, a supplemental safety valve set at 110% MAWP can be installed with only 10% overpressure for fire case. The maximum accumulation in any case can not excced 121% MAWP.