Tailpipe losses not limited to 10%

[don1980]

I've just become aware of the section in API 520 Pt1 (3.3.3.1.3) that allows higher built-up backpressure (higher than 10%) for cases where the overpressure is > 10%. It says that the built-up backpressure can be >10% provided that it doesn't exceed the allowable overpressure.

Technically, this makes sense but I'd never realized that it was explicitly allowed according to API standards. Primarily, i think this has application in justifying existing installations that exceed the 10% limit, but are sized for a fire scenario. It would also apply to process upset cases in which the PSV is set below the MAWP.

I don't recall ever hearing or reading any discussion on this topic. Is this widely practiced? It hasn't been practiced in my company but I think it should be. Do any of you have any comments on this? Thanks.

Here's the whole text of this section:
3.3.3.1.3 In a conventional pressure relief valve application, built-up back pressure should not exceed 10% of the set pressure at 10% allowable overpressure. A higher maximum allowable built-up back pressure may be used for allowable overpressures greater than 10% provided the built-up back pressure does not exceed the allowable overpressure. When the superimposed back pressure is constant, the spring load may be reduced to compensate for the superimposed back pressure. In this case, it is recommended that the built-up back pressure should not exceed the allowable overpressure. When the downstream piping is designed within the above back pressure criteria, no back pressure capacity correction(Kb = 1.0) is required in the valve sizing equations, for gases at critical flow or for liquids. When the back pressure is expected to exceed these specified limits, a balanced or pilot operated pressure relief valve should be specified.

 

[Chance17]

Keep in mind that the API Standards are recommended Practices.
Also keep in mind the 10% backpressure rules for conventional valves are mainly to avoid chatter.
I have seen organizations ignor the 10% backpressure limit for Fire contingency.
The rational is that "who cares about chatter if a Fire occurs".
New construction should always meet the 10% rule.
There may be some forgiveness for older facilities - especially if it can be demonstarted the vessel is not over pressured.

 

[don1980]

I'm surprised that this practice is seldom used. It's safe and is makes sense technically. If the tailpipe losses are above 10% but less than the % overpressure, then valve is no more likely to chatter than it would at 10% overpressure.

If the tailpipe losses are say 15% while the overpressure is 21% (fire), then the PSV is less likely to chatter as compared to the case where the tailpipe losses and overpressure are both 10%. For stability, the standard rules (3% & 10%) result in the pressure drop across the PSV equal to 97% of the set pressure. As long as the tailpipe losses are < the overpressure, that minimum value (97%) is satisfied.

The only hazard is that this practice will be misapplied, but that's why relief designers need to be well trained. There are countless opportunities for a relief design to be botched by an insufficiently engineer.

 

[david6245]

For my clarification, would the following example be acceptable for an existing installation per this section of API 520:

Vessel MAWP = 150 psig.
PSV set at 100 psig that vents to the atmosphere.
Pressure drop in the inlet line at PSV capacity is 10% or 10 psig.
Pressure drop in the outlet line is 15% of set pressure, or 15 psig.

So the pressure profile of the system would be: 125 psig (at vessel) ->115 psig (at inlet of PSV)->15 (at outlet of PSV) -> 0 psig at outlet of vent line.

Since the resulting backpressure calculated at PSV capacity does not overpressure the vessel above its MAWP, I would reason that the pressure drops are acceptable.

Your thoughts?

 

[lizking]

1. API 520 is now titled a "Standard." (Eighth Ed.)
2. Keep in mind that if your discharge dP meets your overpressure at values above 10%, it may not meet it at 10%. All the cases have to be checked. (has to do with the RV capacity being directly proportional to relieving pressure; but dP is proportional to the square of the flow)

 

[skearse]

I would check with a valve vendor but I don't know that any of them would consider that 'acceptable' since the 15% dP may likely cause the PSV to chatter...I don't know that I'd use a conventioonal valve in that application.

 

[dcasto]

Think about this; On gases going through an orifice (a psv), if the pressure drop is 10% of the inlet, what is the maximum flow?, Now repeat with 20% to 90%. Yep, criticle flow sets in at about 50% it it doesn't matter what the outlet pressure is, the psv will not relieve any more volume at 50% than 90%.

SO David6245's set point of 100 psig psv will be at 100% open at 110 psig and the vessel will have 9% more due to losses and hence 119 psig on it. The outlet pressure can be as high as 60 psig and the PSV will flow the same as if it had 0 psig.

 

[EGT01]

dcasto,

If your relief system was only a PSV nozzle, then I would agree that for critical flow conditions, the back pressure has no affect on the PSV capacity.

However, a PSV has a disk that is affected by both the amount of pressure on the upstream and downstream side of the disk. For conventional style relief valves, the amount of disk lift is adversely affected when the back pressure exceeds the amount of overpressure. At small differences, valve operation may be stable but capacity will likely be reduced. At larger differences, not only is capacity further reduced but valve operation can become unstable resulting in the valve chattering.

Here's an excerpt from the Anderson Greenwood Technical Seminar Manual. The attached file is the figure 3 mentioned below.

http://files.engineering.com/getfil...4824-a01f-31f975def310&file=backpressure2.jpg

The ability of a typical conventional PRV to tolerate the
effects of built­up back pressure with 10% allowable
overpressure is illustrated in Figure 3. The shape of the
curve readily shows why API RP520, ASME Section
VIII, and most (if not all) other codes of good practice
recommend a maximum built­up back pressure on a
conventional PRV of 10%. There is a slight loss of lift
and capacity beginning at about 8% back pressure,
rapidly increasing with higher back pressures until the
PRV shuts prematurely, usually re­opening immediately
and repeating the cycle very rapidly, normally referred
to as ``rapid cycling'' or ``chatter''. Any PRV inlet
piping pressures losses will aggravate the possibility of
PRV chatter, when combined with the effects of built­up
back pressure.

The capacity of bellows type valves can also be affected by back pressure before reaching sub-critical conditions and require the use of a derating factor in the valve sizing equation.