Did you know, the allowable total backpressure for a conventional valve can be larger than 10% of the set pressure? According to API 520-1 §5.3.3.1.3:
Many people think a relief valve on a pipe should never see more than 10% backpressure for spring-loaded relief devices. In reality, that’s a guideline for normal cases, not a hard law of physics. That 10% is not a one-size-fits-all limit—it depends on how much extra pressure (overpressure) the valve can handle during an emergency or extreme case.
According to API 520-1 §5.3.3.1.3, “in a conventional PRV application, when the allowable overpressure is 10%, the built-up backpressure should not exceed 10% of the set pressure. A higher maximum allowable built-up backpressure may be used for allowable overpressures greater than 10%, provided the built-up backpressure does not exceed the allowable overpressure.”
So, what does this mean, and when is it applicable? In this blog, we’ll go over a few examples and explain the answers to these questions in simple terms.
What is the Allowable Backpressure Myth?
It is traditionally accepted that a conventional spring-loaded relief valve should have no more than 10% of its set pressure as backpressure—pressure in the outlet line—to avoid instability. For example, if a conventional relief valve is set at 100 psig, back pressure should stay equal to or below 10 psig. While this is a good general guideline to keep valves working properly in many normal scenarios, the myth arises when we assume that the guideline is applicable in all situations and with all valves.
What is the 10% Guideline Variable?
The allowed backpressure for any system is tied to how much overpressure the valve or system can tolerate. If a scenario allows the valve to go 10% over its set point, which is common for single valve, non-fire cases, then 10% backpressure is the recommended cap. However, if the scenario is one where more overpressure is permitted (such as 21% in a fire), then a proportional amount of additional backpressure is acceptable, up to that same percentage.
Essentially, in emergency situations that already allow higher pressure in the system, the valve can handle more backpressure. We’ll go over some of those situations now.
Example: Fire Cases
In a fire scenario, codes allow a vessel’s pressure to rise 21% above Maximum Allowable Working Pressure (MAWP). In that case, a conventional Pressure Safety Valve (PSV) on that vessel can have up to 21% built up backpressure and still function properly. So, for a PSV set at 100 psig , backpressure could be 21 psig (instead of the traditional guideline of 10 psig). This is because, due to the fire emergency, the valve is allowed to lift later at higher pressure, and it won’t choke or flutter as long as it stays within that 21%.
Example: Multiple-Valve System
If two valves are protecting the same system in a situation wherein one opens slightly later than the other, the one set higher (5% over the MAWP of the equipment) would be at 10% overpressure to stay within an overall 16% accumulation for two valves. This means that the first valve might allow ~16% backpressure, and the second only ~10%.
For instance, if Valve A is set at 100 psig (MAWP is also 100 psig), it could see 16 psig backpressure, while Valve B, set at 105 psig, could see 10 psig backpressure. This is because 105 + 10 ≈ 115, aligning with 116% of the MAWP allowed for two valves. These numbers come from API guidelines and ensure neither valve reaches levels above its stability range.
Below is a table outlining these examples:
Other Factors to Consider
Keep in mind that there is no capacity correction needed if backpressure is within the allowable ranges tied to overpressure. As long as backpressure is within range, the valve will flow at rated capacity without issues. It’s only when those limits are exceeded that you may need to consider reducing the capacity or worry about instability.
That being said, lowering a system’s set pressure can also help mitigate outlet backpressure concerns without the need to replace valves or piping. Additionally, it’s important to consider other equipment within the system when allowing for higher overpressure. There are situations when the set pressure is set to a value below the MAWP to protect downstream equipment. In this case, the allowable overpressure will depend on all of the protected equipment.
When Should You Switch Valve Types?
If the expected backpressure for any scenario will be higher than the allowances we’ve outlined above, then it is recommended to use a balanced-bellows PSV or a pilot-operated PSV. These valve designs can handle high backpressure without malfunction. Additionally, note that some conventional valves have an open bonnet or vented spring design, which may allow the valves to tolerate backpressure differently. However, even then, the safest approach is to consult with the manufacturer for any special cases.
Understanding Backpressure Guidelines
The common 10% backpressure rule of thumb is a good guideline for typical scenarios, but it is not a strict cap in all circumstances. The allowable backpressure equals the allowed overpressure for the event (capped at whatever code allows for that situation). Understanding this helps prevent over-engineering, as you might not need to redesign piping or switch to a balanced valve if your backpressure, although above 10%, is still within code allowances for that event. Conversely, it prevents complacency—if you are beyond those allowances, you need a different solution.
If this sounds complicated, don’t worry! The key message is: check what scenario you’re in. If it’s a severe scenario like a fire or multiple-valve situation, the relief valve has more wiggle room for backpressure. If you’re ever beyond that wiggle room, then you know it’s time to use a more specialized valve design.
For your reference, we have included additional tables with more in-depth scenarios and variables. You can find these below.
Single-Valve Installation:
Case 1: PSV Set at MAWP
Case 2: PSV Set Below MAWP
Case 3: PSV Set Pressure Compensated for Constant Backpressure
| Relief Variable | Unit | Case 1 | Case 1 Fire |
Case 2 | Case 2 Fire |
Case 3 | Case 3 Fire |
| Constant Backpressure Pcon |
psig |
0 | 0 | 0 | 0 | 25 | 25 |
| CDTP PCDTP |
psig |
100 | 100 | 90 | 90 | 75 | 75 |
| Set Pressure Pset |
psig |
100 | 100 | 90 | 90 | 100 | 100 |
| Allowable Accumulation AAP |
% |
10% | 21% | 10% | 21% | 10% | 21% |
| Maximum Allowable Working Pressure PMAWP |
psig |
100 | 100 | 100 | 100 | 100 | 100 |
| Allowable Overpressure AOP |
psi |
10 | 21 | 22.2 | 34.4 | 10 | 21 |
| Allowable Backpressure PB, Allowable |
psig |
10 | 21 | 20 | 31 | 10 | 21 |
| Allowable Backpressure % %PB, Allowable |
% |
10 | 21 | 22.2 | 34.4 | 10 | 21 |
Multi-Valve Installation:
Case 1: 1st PSV Set at MAWP, 2nd PSV Set > MAWP
Case 2: 1st PSV Set < MAWP, 2nd PSV Set > 1st
Case 3: PSV Set Pressure < MAWP and Compensated for Constant Backpressure
Case 4: 2nd PSV Set > MAWP (Fire Only)
| Relief Variable | Unit | Case 1 | Case 1 Fire |
Case 2 | Case 2 Fire |
Case 3 | Case 3 Fire |
Case 4 Fire |
|
Pcon |
psig | 0 | 0 | 0 | 0 | 25 | 25 | 0 |
| P1,CDTP | psig | 100 | 100 | 95 | 95 | 70 | 70 | 100 |
| P2,CDTP | psig | 105 | 105 | 100 | 100 | 75 | 75 | 110 |
| P1,set | psig | 100 | 100 | 95 | 95 | 95 | 95 | 100 |
| P2,set | psig | 105 | 105 | 100 | 100 | 100 | 100 | 110 |
| PSV 1, AOP | % | 16% | 21% | 22.1% | 27.4% | 22.1% | 27.4% | 21% |
| PSV 2, AOP | % | 11% | 16% | 16.8% | 22.1% | 16.8% | 22.1% | 11% |
| AAP | % | 16% | 21% | 16% | 21% | 16% | 21% | 21% |
| PMAWP | psig |
100 |
100 | 100 | 100 | 100 | 100 | 100 |
| AOP | psi |
16 |
21 | 21 | 26 | 21 | 26 | 21 |
| Max P2,set | psig |
105 |
105 | 105 | 105 | 105 | 105 | 110 |
| PB,1,Allowable | psig |
16 |
21 | 21 | 26 | 21 | 26 | 21 |
| %PB,1,Allowable | % |
16% |
21% | 22.1% | 27.4% | 22.1% | 27.4% | 21% |
| PB,2,Allowable | psig |
11 |
16 | 16 | 21 | 16 | 21 | 11 |
| %PB,2,Allowable | % |
10.5% |
15.2% | 16% | 21% | 16% | 21% | 10% |
