9 replies to this topic

[ryn376]

If we have a have a hydraulic expansion case where the fluid can be heated above it's normal boiling point when blocked in an exchanger, do we need to size the relief valve orifice for two phase flow or consider a two phase flow discharge line pressure drop? Hypothetically, let's say cooling water is on the cold side of an exchanger (assume set pressure of 150 psig) and can be heated to 250 F by the process stream if blocked in, would anybody carry out a two phase flow orifice calc? What about two phase flow in the discharge pipe? Thank you.

Edited by ryn376, 24 August 2018 - 05:59 AM.

[SawsanAli311]

Hi ryn, 

 

As per shell dep 80.45.10.11, thermal relief scenario is a vapor only relief scenario and it is considered when the vapor pressure reached from the blocked in heating condition exceeds the design pressure of the cold side. There is no mentioning for a two phase relief for this scenario.

In this case, you need to calculate the relieving temperature and decide if the scenario is credible by comparing to the maximum operating temperature in the system. If credible, you then perform several multi flashing cuts until you reach the relieving temperature and in each cut you find the latent heat.  for each cut, use the exchanger's design heat duty to estimate the required vapor relieving rate and size the PSV. Select the cut with the maximum PSV area.

[Bobby Strain]

Shell's DEP is not a recognized code by any means. It's intended to be used only for Shell projects. So, I would wait for some more responses.

 

Bobby

[SawsanAli311]

Hi Mr.Bobby, We tend to follow shell DEPs in our projects in my company (which is not shell), I would be also interested to get to know what other codes  as well. 

However, as per API 521 thermal relief can be assumed as vapor only: ''If the blocked-in liquid has a vapor

pressure higher than the relief design pressure, then the PRD should be capable of handling the vapor-generation

rate. If discovery and correction before liquid boiling is expected, then it is not necessary to account for vaporization in

sizing the PRD.''

Edited by SawsanAli311, 24 August 2018 - 01:39 AM.

[ryn376]

SawsanAli,

 

The API 521 quote on thermal relief is a helpful note. I know I must have read that before, but I unfortunately must have forgotten it. 

 

That section makes me think that two phase relief and/or pressure drop should be considered, though API does not explicitly state it in the hydraulic expansion section. I feel (maybe incorrectly) that most people ignore the possible two phase flow and I am just curious if it is overkill to evaluate two phase flow?

Edited by ryn376, 24 August 2018 - 08:45 AM.

[SawsanAli311]

Hi Mr.Ryn, 

 

I presume that if we consider two phase flow, the required PSV area would be relatively greater than the case of vapor only relief (kindly please correct if I am wrong). We need to look also at the aspect of avoiding the oversizing of the PSV in order to avoid chattering i.e. specially when the relieving rate becomes less than 25% of the rated capacity. 

[Bobby Strain]

ryn,

    If the blocked-in container is filled with liquid, any vapor formation will immediately raise the pressure, and the vapor will condense. Thus, you cannot form vapor in such an environment. I don't know what API has to say on the subject, but they are not always right. Whatever guide they develop is the consensus of a group. Not always right, but popular.

 

Bobby

[ryn376]

Is the general consensus that it would not flash and be two phase or vapor in the outlet?

[Art Montemayor]

ryn376 and SawsanAli311:

 

Bobby Strain is absolutely correct in his evaluation and advice given on this topic.  His comments can be proven by inspecting a standard Mollier or T-S chart on the liquid fluid involved.  What he simply states is also common sense.

 

Ryn376 states that his exchanger is having “a hydraulic expansion case where water can be heated above it's normal boiling point by steam when blocked in”.  He then  asks: “do we need to size the relief valve orifice for two phase flow or consider a two phase flow discharge line pressure drop?”

 

Both of you are dwelling in a mix up of what API 521 is covering and discussing.  You fail to understand the actions of the relief taking place due to an over-pressure.  As Bobby has pointed out, the system under hydraulic pressure is 100% filled (as defined).  If you attempt to “boil” this water to steam, you must have a space to accept the steam as the hydraulic pressure increases.  Since you don’t have that expansion space, your system will remain as high pressure liquid at the developed pressure.

 

The result of the hydraulic over-pressure will be that your thermal relief valve will open and high pressure LIQUID will flow through the valve’s trim and instantly expand into a 2-phase mixture AFTER exiting the valve’s orifice.  The valve’s outlet (whether atmosphere or exit piping) will be what has to withstand the expanded 2-phase flow - NOT THE VALVE PROPER.  Consequently, any relief valve outlet piping has to take the 2-phase flow into consideration - not the relief valve itself.   Please read API 521 carefully and understand what is being explained.  I am furnishing you with a copy of my 5^th Edition as attached.  Read it carefully and you will not be surprised to find that when you submit your relief valve datasheet to your valve supplier you will probably be told that a conventional ¾” -1” valve size will do because of the small liquid quantity exerting the hydraulic expansion.

 

SawsanAli311:
When you quote any Engineering Standard or code to an engineer, always cite the Edition or version number.  As Bobby has correctly pointed out to you, the API is dictated by human beings - and they inevitably make mistakes, omissions, and errors and for that purpose always upgrade their standards.  So please be specific.

 

Ryn376:

The answer to your question is that you will have mixed flow exiting the valve and only high pressure liquid entering it.  Handle the exit flow appropriately.  And study up on Phase Equilibria.

 

 Hydraulic Pressure Relief - API 521 5th Ed..docx   101.44KB   51 downloads

[SawsanAli311]

Thank you Mr.Art for the thorough explanation