Strategy for Evaluating PRV Options on Secodary Crude Column

[UtahUte60]

Hello Fellow Professionals:

I have a question regarding the strategy to use to evaluate three different options for PRV'S on a secondary crude column which currently has 5 PRV's on the OH line to a condenser. The issue to resolve is that the inlet pressure drop for the current arrangement does not meet th 3 % rule.

the options are as follows:

1.) Replace all existing (i.e. smaller) 5 PRV's with three 8T10 PRV's one on the existing 18" nozzle on the top of the column, one on a side stripper coloumn (Part of the protected system) and one on a 14"
line off the main secondary column crude OH line.
2) Relocate one of the existing PRV's to the the 18" nozzle on the top of the main column along with the existing PRV, and relocate one of the existing PRV's to the top of the side stripper column, and upsize the existing 14" line off the main OH line to 18" so that it contains the remaining 2 PRV's.
3.) Same as two above except that that three of the PSV's on the main secondary crude column OH line are pilot operated PRV's.

Regards:

UtahUte60

 

[georgeverghese]

a)For each of these options, are these PSV's in the revised configuration all set at the same pressure? Do you not have to reduce the setting (or the usual 10% overpressure limit ) on the PSV at the downstream stripper so that relieving pressure seen at the sec column remains within design pressure + 10%?
b)Are you sure you meet the 3% inlet loss limit for the PSV on the side stripper for its rated flow?
c)For option 3, as far as I know, there is no dispensation permitted to this 3% inlet loss rule for pilot operated PSVs.

 

[UtahUte60]

georgeverghese:

Siemens recommended that all the PSV's be set at 30 psig, but other have recommended staggered settings of 5 % difference in set point. The downstream stripper is actually a steam stripper connected to the crude column. I need to check the 3 % rule on all the PSV's. As far as the pilot operated relief valves I am not sure this is a clean service anyway. Thank you for your response.

 

[georgeverghese]

Agreed, a lower setting for the PSVs' on the steam stripper top seem appropriate.
There is a well known major disaster event ( believe this was BP Texaco refinery) where the PSV's on a column OVHD line were located at a low point in the piping to the condenser - hope your PSVs' are at a high point on this OVHD line.

 

[don1980]

At low set pressures, like the case we're discussing, it's inherently more difficult, and sometimes impossible, to satisfy the 3% rule. 3% of a small number is a very small number.

When you have problems meeting the 3% rule, consider these two low-cost options (listed below) before you consider more costly field modifications. The first one is actually a no-cost option, aside from the cost of doing the engineering analysis.

(A) Engineering Analysis (API 520 Pt II, 6th ed., 7.3.5 - 7.3.6): This is an equally acceptable and compliant way to evaluate PRV stability. It's an alternative to the legacy 3% rule. If the installations pass the Engineering Analysis, then no changes are needed. Obviously this is the first option to consider. You said you're working with Siemens on this design. I know that Siemens is knowledgeable about this API 520 Engineering Analysis procedure. Ask them to perform this analysis to see if the existing installations are acceptable.

(B) Restricted Lift (API 526, 7th ed., Annex E): By restricting the stem lift, the PRV's rated capacity (the capacity used for pressure loss calculations) can be reduced to a user-defined value which is high enough to satisfy the required flow but lower than the rated flow of the existing orifice (at full lift). There's a worked example problem in Annex E that illustrates the usefulness of this design option. If a lift restricter can be installed in your existing PRVs (check with manufacturer), then the cost of this solution is very low.

And, here are some comments on the three alternate designs you listed:

(1) The larger valves, such as the 8T10 valves mentioned, are more likely to experience inlet pressure drop problems. That's because the valve body area (for API 526 PRVs) doesn't increase in proportion to the increased area of the orifices which fit into these valves. Essentially, the larger API 526 PRVs are too small for the orifices that fit into them. Thus, these larger PRVs are more prone to experiencing inlet and outlet pressure drop problems.

(2)Insufficient info about this option. There isn't enough information to allow for informed comments.

(3) You didn't specify whether you're referring to integrally sensing pilot valves, or remote sensing pilot valves. Remote sensing pilot valves will definitely solve your inlet pressure drop problem, because the pressure drop has no affect on the stability of the valve. High inlet pressure drop has no affect on the stability of the valve. That's because the pilot is sensing the pressure inside the vessel rather than the pressure at the inlet to the PRV.