5 replies to this topic

[Brian Hughes]

Hello all,

I have a question regarding the calculation of percent pressure drop (%dP) for the 3% rule/guideline for relief system inlet piping pressure drop per API / ASME. I found this post from a few years ago that is directly related to my question, but it doesn't seem to me that the thread came to any definitive answer, so I am posting the question again.

My question is, after you calculate your inlet piping pressure drop, do you divide that by the set pressure in gage units or absolute units to get the %dP for comparison with the 3% rule?

For certain applications, this could mean the difference between meeting the 3% pressure drop requirement and not. I realize this 3% requirement is not mandatory, and one can use the manufacturers recommendations. But, for all intents and purposes of this post, let's assume it is a mandatory requirement.

So, for example, let's say I have a relief system with a set pressure of 75 psig and I have calculated the pressure drop across the inlet piping to be 2.5 psi. Then we would have the following calculations, one with gage units and the other with absolute units:

Pset = 75 psig = 89.7 psia

dP = 2.5 psi

1. %dP = (dP / Pset) * 100 = (2.5 psi / 75 psig) * 100 = 3.33%

2. %dP = (dP / Pset) * 100 = (2.5 psi / 89.7 psia) * 100 = 2.79%

Now, engineering judgement would tell me that the more conservative case would prevail, i.e. I should use the set pressure in gage units since it will always be smaller than absolute units and therefore yield a higher %dP.

However, I would like to know what standard industry practice is for these calculations, and what the good people of cheresources practice.


Summary Questions:
To calculate percent pressure drop for comparison with the 3% rule per API / ASME, does one use the set pressure in gage units or absolute units?
What is the standard industry practice for calculating percent pressure drop with regards to the above question?


A little about me:
I am a young mechanical engineer working in the chemical industry. I only have about a year and a half of working experience, but I have been heavily involved with relief systems for the past 6 months or so.

Also, I apologize if this post is a bit wordy, but after reading a lot of other posts here at cheresources.com, I wanted to be sure I was as clear as possible about what it is I am looking for.

[flarenuf]

Brian
Flare systems are almost always worked in gauge pressure. So 3% is of set pressure in barg and the 10% maximum backpressure criteria is also based on gauge set pressure .

brian

[skearse]

I'll chime in here to update my post from that thread. Still agree that it's calculated based on the gauge set pressure basis (in practical terms, to prevent chattering of the valve).

I'll change my stance on the 3% being "non-mandatory" since I've heard rumors (from a very reliable source) that OSHA is currently handing out fines for not complying with the 3% rule.

[kkala]

Following is noted in addition to above posts.
Allowable frictional pressure drop (ΔP) to the PSV inlet is 3% of PSV set pressure in gauge units, or lower. This is not only per API / ASME, but also according to other local practices known.
These practices may permit ΔP a bit higher than 3% in specific cases, yet the percentage is based on the PSV set pressure in gauge units.
So reply to the summary questions (post No 1) : comparison to be made with 3% of gauge set pressure (3.33%); this is standard practice (but 3% can differ somehow).
This is by convention, but can be explained by the fact that design pressure is expressed in gauge units (even this is not mandatory, no relevant rule found). It is noted that local pressure gauges show gauge (not absolute) pressure.

Edited by kkala, 09 October 2012 - 12:54 PM.

[Dacs]

This is how I view the topic:

We are discharging our flare at atmospheric pressure so any pressure differential is always based on atmospheric pressure (NOT vacuum).

In essence, it's practical to fix our reference pressure NOT at vacuum, but at atmospheric pressure, which what gauge pressure practically is.

My 2 cents

[fallah]

My question is, after you calculate your inlet piping pressure drop, do you divide that by the set pressure in gage units or absolute units to get the %dP for comparison with the 3% rule?

Brian,

In order to verification of PSV inlet line sizing/configuration, inlet piping pressure drop should be divided by set pressure in gauge unit.

Fallah