9 replies to this topic

[confusedchamp]

Stuck at dead end for high inlet line pressure loss.

 

The relief valve is located downstream of a pressure regulator. The regulator failure is the governing case and  API orifice D is what is required for this case.

 

Now problem is the downstream line size of regulator is 1/4". Considering the standard inlet outlet size of PSV, I need a 3/4" line. Now in calculating the inlet line losses, i have considered the following,

1. Expansion loss from 1/4" line to 1.5" inlet line
2. 1.5" line to 3/4" to PSV inlet.  

 

No matter how much i change the inlet line size, the expansion loss from 1/4" to x" is dominating the pressure drop, and is coming more then 3%. (way beyond 3% upto 20%).

 

i am calculating the line loss for rated flow.

 

Any help to solve this is highly appreciated. 

[fallah]

Hi,

 

The inside cross sectional area of 1/4" line is almost equal to effective orifice area of API orifice D. On the other hand, the maximum PSV inlet size of D designation is 1-1/2" (it can be 1" while you mentioned 3/4" for PSV inlet?!). Then you have to put the branch for PSV installation as close as possible to pressure regulator to reduce friction loss in process path and in branch just consider an expander from 1/4" to 1-1/2" fit to fit with the following PSV. The PSV should be pilot operated type with sensing line taking from the 1/4" line itself in order to cope with the high inlet line loss. After doing so and also rechecking your calculation please let's know the result. 

Edited by fallah, 01 June 2013 - 02:10 AM.

[mansari]

When we talk about Relief Valve Installation (API RP 520 Part II or ASME Sec VIII Appendix M), there is an equally important consideration also for the inlet piping, other than 3% line pressure drop. The codes state, " everything inclusive of line, isolation valve etc must have an internal cross sectional area EQUALor HIGHER than the relief valve inlet. In my opinion, your described piping configuration does not conform to the standard installation practices and need to be modified.

 

If the relief valve capacity was determined based on 100% opening of the pressure regulator, the downstream line should have also been sized accordingly to avoid that excessive pressure drop you have calculated.

[latexman]

Could the regulator be over sized for the application?  If so, an orifice change or smaller regulator may be a good option.

[fallah]

Could the regulator be over sized for the application?  If so, an orifice change or smaller regulator may be a good option.

 

Latexman,

 

Adding to your options: Considering a mechanical stop (if is possible) on existing regulator to prevent regulator to be in full open position due to failure...

[mansari]

I am still trying to understand the rationale for such installation where the relief valve is installed downstream of the pressure regulator. If the regulator fails in close position, there is no point, this relief valve could offer a protection. On the other hand, if regulator fails open (as it is stated), then the discharge would go to the atmosphere anyway. Relief valves are directly installed on piping and vessels, need to be protected,, don't they?

[fallah]

mansari,

 

Just my guess might clarify the matters you stated:

 

1-Upstream piping of the regulator might be designed for upstream full pressure.

2-Downstream line of the regulator might be terminated to a user of, let say, nitrogen, fuel gas, etc...then the pressure could be built up there due to regulator full open condition...

3-Downstream piping of the regulator might has lower rating than the regulator upstream, then need to be protected against overpressure due to regulator full open condition...

 

Now is the time the OP should come back and clarify the mentioned points along with a simple sketch...

[latexman]

Some scenarios get a huge safety and/or environmental benefit by relieving pure inert gas just downstream of failed regulator instead of relieving inert gas laden with solvent, hydrocarbon, monomer, or chemicals similar to "methyl ethyl death" contained in the vessel being protected.

[confusedchamp]

Thanks all... I my big apologies for being away from this post ...The net was down for unusually long and so i could not reply, though i have read the replies on saturday, which helped...  .All your replies have really helped, a big thanks.

 

Mansari, huge thanks to u... i really missed out that point in API. It was the reason no matter what the line size was, the pressure drop was always high. Have asked for the resolution from the system designer. They are working on it. Will post the resolution as soon as i receive.

 

Fallah : The three points you mentioned above are exactly how the system is ..... Will be posting a sketch.

 

Latexman : The regulator is properly sized by the downstream piping is not sized adequately i suppose. Am waiting for reply from system designer on the same. 

[mansari]

Thanks confusedchamp. Yes please do share the sketch with us for our understanding of the system. Regards