4 replies to this topic

[Sunil Pal]

Dear All,

What should be the location of PSV ?
Generally wherever we found two MOV/isolation valves or wherever the chance of liquid get trapped between two valves we put one relief device as a safeguard of the line.
I wanted to know what is the minimum length/relief quantity required between two valves. Sometimes the length is very less like 2, 3, ........ meter, still is it required ? and the same case if the length is very high, like 1000, 2000, 3000, 4000......meter, does the single PSV be adequate ?. If no, than how many PSV's required ?. For the same purpose relief quantity is required as I explained in last post. Withought relief quantity I can not find the exact no. of PSV required. The same is not explained in API also. is there any guideline for that ?.
For the calculation of relief quantity due to thermal radiation only, I am having an exel sheet, which I made considering the things which i explained in last post, please feel free to ask me if needed.

Safety Regards,

Sunil Pal
EEC, Mumbai

[Art Montemayor]

Sunil Pal:

Below are responses to your specific comments and questions.

What should be the location of PSV ?
Anywhere you like – as long as it is between the two shutoff valves?

I wanted to know what is the minimum length/relief quantity required between two valves.
And you were told the answer(s) in your last thread. There is none used in the USA. We simply install a ½” to ¾” thermal relief valve – period! We told you already that calculation of the relief rate is considered a waste of time and is not done for thermal relief valves.

Sometimes the length is very less like 2, 3, ........ meter, still is it required ?
Yes.

and the same case if the length is very high, like 1000, 2000, 3000, 4000......meter, does the single PSV be adequate ?.
Yes – but where the heat is applied.

For the same purpose relief quantity is required as I explained in last post. Withought relief quantity I can not find the exact no. of PSV required.
You were given our responses. Apparently, you didn’t like them. In fact, you were given an equation for an approximation of the relief rate. Why don’t you use that and generate your needs?

The same is not explained in API also. is there any guideline for that ?
You were explained all that in the last thread. Again, if you are not satisfied with the responses because you don’t like them or you refuse to accept them. API takes the same approach as we explainded to you: no relief rate is calculated.

[vinay]

Sunil,

As Mr. Art has explained that there is no point is calculating Thermal Relief Valve Capacity , but to specify the minimum 1/2" to 3/4" valve with maximum orific area permitted in that size valve.

However to satisy yourself , let me tell you how to calculate the thermal relief requirment due to solar radiation based on basic heat balance.

Lets say you have to calculate the thermal relief requirement on "L " Meter piping between two block valves which is holding some process liquid.

First calculate the volume of liquid between these two block valves, which will be

V= pi()/4 * d2 * L -- assuming piping is full of liquid

where "d" is pipe diameter

calculate mass of liquid by multiplying volume with density of liquid at initial temperature

m = V * DENSITY

calculate the heat transfer rate to pipe fluid from solar radiation / fire case. "Q" Kcal/hr

calculate temperature T2 from Q = m Cp (T2-T1)
where cp = sp. heat , T1 = Initial temp of liquid in pipe ,

calulate the density of liquid at T2 temperature

calculate the volume of liquid trapped in pipe at temperature T2 ,

V2 = m/Density at T2

now V2-V is rate at which PSV needs to discharge to maintain pipeline pressure , otherwise pipeline will pressurise. Usually very small number & hence even 1/2 " or 3/4" size PSV is all it requires.

I hope this info will be of help.

Regards,

Vinay Mathur

[JBradley]

Ummm I may have missed something here because you guys are referring to a previous thread which I haven't read.

Anyway my question is - are you saying that for relief lines, you don't bother calculating the requisite discharge area in the USA????

If this is the case then I'm from the UK and can probably give you a different answer - hey maybe one you'll like

[pleckner]

@JBradley:

To answer your question, for the most part, and there are exceptions, one does NOT need to calculate the relieving rate for these type of relief valves (OK, at least in the U.S.). You should read the previous thread to see why.

And yes, speaking for myself (and probably a whole lot of other people too), I would love to see your different answer!!! That is what these Forums are all about, different ideas, information and opinions.