inlet piping hydraulic calcs 2

[jrwchem]

Hi,

My question is when calculating the 3% loss in the inlet pipe from the vessel to the psv inlet should the pressure of the vessel be assumed to be at the set pressure or the set-pressure+10% (to account for overpressure)?

For example if the set pressure is 100 psig. then i think i should set the vessel pressure at 100 psig, and ensure that the losses to the psv inlet don't exceed 3 psig .

Or if i set the vessel pressure to 110 psig, do the hydraulics, and ensue the losses dont exceed 3%.


However, i think that "they say" that the psv does not achicve full rated flow until 10% overpressure. So the PSV would not be fully open until the pressure of the psv inlet was 110 psig. However this means that the pressure of the vessel will be greater than 110 psig due to pressure loss. But, by definition the overpressure is the maximum allowable vessel pressure. So by my (flawed??) logic this can not be!

Can anbody set me straight on this?

Does anybody want to comment on the fact that the pressure loss is no-recoverable, so we should consider changes in dynamic pressure and not static pressure?

Thanks.

Jonathan

 

[EGT01]

jrwchem,

It might help to think about the general procedure for designing a relief system. First, determine the required relief rate and then select a relief valve size with a greater capacity. Next, size the relief system piping for the relief valve you have selected, keeping in mind that you have a relief valve with a capacity greater than what you need. In one respect, you can think of these as two independent steps.

As an example, if your relief valve is set at the vessel MAWP and the required relief rate is determined with the vessel at 10% accumulation, then if you have selected a relief valve of greater capacity, you will not really be at a full 10% overpressure at your relief valve inlet. This will not be a problem as long as you have not grossly oversized your relief valve.

Regardless that your required relief rate is less than the valve capacity, the 3% inlet line pressure drop rule comes from ASME Section VIII, Div 1, Appendix M and is also found in API RP-520 Part II and is based on the rated capacity of the relief valve. This is the general requirement for sizing of the inlet piping and should be followed to ensure that the inlet piping does not adversely affect the operation or capacity of the relief system unless a more detailed engineering analysis is performed.

The only time you would have the problem you describe would be when you select a relief valve that has a certified capacity equal to (or nearly equal to) your required relief rate. So keep that in mind when selecting your relief valve size.

Regarding the non-recoverable losses, again see the above industry references. The 3% inlet loss is related only to non-recoverable losses. Any difference in static pressure between the relief valve and the equipment it is protecting needs to be accounted for by selection of the relief valve set pressure. For example, if a relief valve is located above a vessel with MAWP = 100 psig and there is a 10 psi static head difference between relief valve and the top of the vessel, the relief valve set pressure should not be greater than 90 psig.

 

[RJB32482]

EGT01,
So say I have a relief valve set at 125 psig and the valve is 3 feet in the air. Even if I include the static head in the pressure drop calculation, and its under the 3% of set pressure, do I need to drop the relief valve to 122 psig?

Also, I believe 10% accumulation is for outlets of P.D. pumps, correct?

Thanks

 

[EGT01]

RJB32482,

For a vessel constructed to ASME Section VIII, Div 1, I would refer you to section UG-134(a) and UG-134(c) which basically says when a single pressure relief device is used, the set pressure shall not exceed the MAWP of the vessel and the set pressure shall include the effects of static head and constant back pressure.

If in your example you have an overpressure event that involves a fluid where 1 foot of fluid is equal to 1 psi static head, then yes, the set pressure of your relief device must be reduced to 122 psig if it is 3 feet above a vessel which has a MAWP of 125 psig and which has been designed to account for the vessel full of such fluid. If the vessel design has not accounted for a fluid with that density, then additional reduction in set pressure will be required to address an overpressure event involving the high density fluid.

Your question about accumulation is starting to deviate from the topic for which jrwchem has started the thread so it may be more appropriate for you start a new thread for further discussion. However, I will say that the allowable accumulation is a function of the design allowances for the equipment you are trying to protect and there are some differences between pressure vessels and piping. As for the pump itself, you should check with the pump manufacturer for any other limitations.