Chemical & Process Technology

I am working on an offshore flare revamping project. With new conditions, one of the existing Pressure relief Valve (PSV) outlet experiencing Mach no of 0.85 which is higher than the maximum allowable Mach no of 0.8 in the COMPANY 's General Practices. Should a Mach no of 0.85 is acceptable ?

Should we size Pressure Relief Valve tail pipe, lateral & main header at choked (Mach no =1 ) ?

Above question have been in my mind since many years ago and they have been raised on many occasion. Simple answer to above question is YES & NO.

YES if ...

Mechanical Integrity
Sizing PSV tail pipe, lateral & main header for choked flow is not an issue and it has been done it for many projects. NEVERTHELESS, this kind of design required proper analysis and serious consideration on Flow Induced Vibration (FIV), Acoustical Induced Vibration (AIV), dynamic stress analysis, welding type analysis, selection & implementation, stringent implementation procedures, 100% x-ray test, etc. The PIPING shall be fit for the stress induced by the flow at Mach 1.

Structure Integrity
Several vibration on piping with fluid flow at choked condition may transmit to support structure and potentially lead to structure failure. Again, the structure supporting piping experiencing choked flow shall be properly analyzed and fit.

Tolerable Flow Measurement
Flow meter may be installed on the main header to measure quantity of inventory release to atmosphere. Some flowmeter like ultrasonic flowmeter which is a good choice for flare metering as discussed in Use Ultra-Sonic Flowmeter in FLARE Gas Header for emission monitoring. Severe vibration may lead to inaccurate measurement when it is at choked condition.

Proper Selection of PSV
The product of Mach 1 would be high pressure drop along the line and results high guilt-up backpressure to the PSV. The design shall ensure that the capacity selected PSV will NOT be affected by the high built-up back pressure at the PSV outlet e.g. balanced type PSV. Read more in Several Impacta of Backpressure on Conventional PRV.

Noise Abatement
Another serious issue shall also to be considered is the potential noise level generated by the piping flow in Mach 1 condition. In many event, the noise generated would be higher than the limit impose by local regulatory requirement and company personnel health policy. This would properly lead to requirements of acoustic insulation of piping. As guideline, the noise level limit is about 115 for short term exposure.

A very typically example of above application is the used of SONIC flare tip. Sonic flare tip constructed from PIPE, in single pipe or multiple pipe. It has been designed to create SONIC flow at flare tip in order to gain the proper mixing and elevated flame center.

Others which i have missed out here
If you have any, let me know...


NO as ...

Stringent requirements and criteria
All above mentioned factors shall be properly analyzed and meeting all stringent criteria

Uncertainties in pressure drop estimation

As PSV discharge could lead to two phase flow (certain part within the collection system), the pressure drop may not be estimated correctly. Those the backpressure at the PSV discharge could be uncertain. This is one of the calculation weakness which is hard to be avoided with present affordable technology and tools.

Conservative Approach
PSV is one of the ultimate protective device within a overpressure protection system. It is normally discharged in to the flare collector. As it is the last line of defense within a safety system, thus it is common that corporate management and company policy imposed extra conservatism in the safety related system.

Thus, in conclusion, a PSV tail pipe, lateral & main header can be designed choked flow condition provided that it is properly analyzed and sufficient design margin has been built in to cater for uncertainties.