Relief valve inlet arrangement query 3

[chemks2012]

Dear all

Can I install an expander at the inlet of the relief valve?

Say existing nozzle is 3/4" (assume this the only available nozzle) and the new required valve is 1.5" (orifice F) and the valve is set at 6barg for the valve maximum capacity of 0.25kg/sec of saturated steam.

At the valve inlet, there is only an expander and nothing else and the estimated pressure drop at the valve inlet is 1% only (less than 3%) at valve's maximum capacity.

I have never seen expander at the valve inlet and hence this query.

Thanks
KS

 

[pennpiper]

chemks2012,
You may consider this a minor technical point but, there is no pipe fitting called Expander. You can't find it in any Vendor catalog or on the shelf of any supply house.

All piping fittings that change the line size from one size to another are called a Reducer. If I were a piping supply house employee and I got a request for an 'Expander' I might take advantage of you and charge 3 times the normal cost because I could and you obviously don't know the difference.

Now there is no problem changing the line size (larger) from the small vessel nozzle to the larger relief valve size. However, you should increase the line size as close to the Vessel nozzle as possible and make all of the line from that point to the RV the larger size.



Sometimes its possible to do all the right things and still get bad results

 

[EmmanuelTop]

pennpiper is correct. The nominal size of the inlet piping must be the same as or larger than the nominal size of the pressure relief valve inlet connection (API 520/II, 4.2.2), hence the reducer should be installed as close to the vessel nozzle as possible.

Make sure you design the inlet piping as self-drained, which is particularly important in steam service.

Dejan IVANOVIC
Process Engineer, MSChE

 

[chemks2012]

Thanks Pennpiper and Dejan

As quoted by Dejan, API suggests inlet pipe must be equal to larger than the the valve installed. Here API says 'must'. It seems that i can't install a reducer even if my inlet losses are less than 3%. Is that correct? Also I have never seen installation with reducer at the valve inlet and hence I am confused.

 

[pennpiper]

chemks2012,
We seem to have a problem with communication:
You wrote:
"existing nozzle is 3/4" (assume this the only available nozzle)"
and
"the new required (PSV) valve is 1.5" (orifice F)

From this I see:
a) you have an existing Vessel with an existing 3/4" Nozzle and you cannot change that
(tell us if this is right or wrong).

Your calculations require you to have a 1-1/2" PSV. (Is this correct?)
and you will have some unknown length of pipe between the vessel nozzle and the location of the PSV. (Is this correct?)

I recommended you add the reducer (for the pipe size increase) as close to the Vessel Nozzle as possible and then make the rest of the line 1-1/2" to the location of the PSV. Therefore you have only a minimal amount of 3/4" pipe to create a restriction of flow.

Do you understand this?


Sometimes its possible to do all the right things and still get bad results

 

[chemks2012]

Hi Pennpiper,

Thanks. Your understanding is correct and I see where you are coming from.

However, I just want to know how much standard one could follow under such situation where one condition (inlet pressure loss of less than 3%) can be fulfilled while another (i.e. A nozzle MUST be equal or larger than a inlet of a valve) cannot be met?

 

[georgeverghese]

"Relief valve inlet" need not be interpreted as vessel RV nozzle size.

The fittings losses must also include that for the 3/4 inch exit ( exit from large vessel to 3/4inch pipe)- that will burn a lot of pressure in your case.

 

[chemks2012]

georgevergese

Vessel diameter is 1m and the relief valve nozzle is 3/4".

I have taken in to consideration the nozzle/line exit losses. However it seem you are saying I also need to consider losses across a big reducer of 1m X 3/4". Is that what you are saying? Please clarify.

Thanks
KS

 

[chemks2012]

Dear all

I just want to know if anyone of you had an experience of installing a reducer (to enable instaation of a bigger valve on a small nozzle)at the inlet of pressure relief valve where you have met the condition of inlet losses of 3% please?

Thanks
KS

 

[georgeverghese]

If you have accounted for the nozzle exit loss, ok.

 

[e43u8]

chemks2012,

Per the API 520 part II, size of inlet piping including valves and fittings shall be the same as or larger than the nominal size of the PSV inlet connection; vessel nozzle is to be considered as a part of the inlet line. Then, regardless of 3% rule for inlet pressure drop is met or not, all parts of the inlet line from vessel nozzle to PSV inlet nozzle can't have a size lower than that of PSV inlet nozzle...

 

[EmmanuelTop]

See the section "Improper installation", paragraph "Inlet restriction" on page 5 of the document

http://smithburgess.com/wp-content/uploads/2012/07/Smith-Burgess.-Relief-Device-Inlet-Piping.pdf

As long as the minimum inlet line area (3/4" piping) is higher or equal to the PSV orifice area (F= 0.307 in[sup]2[/sup]), the PSV inlet arrangement is considered acceptable. If you know what is the schedule of the 3/4" pipe you can calculate the flow area and if not smaller than 0.307 in[sup]2[/sup], you are probably OK with what you have.

As engineers we should also try to be practical as much as possible, if this does not violate the code(s). Drilling another 1.5" nozzle on the vessel for a 1.5" PSV is a painful job with a lot of paperwork. You won't do it unless you really have to do it.

Dejan IVANOVIC
Process Engineer, MSChE

 

[EmmanuelTop]

See

http://www.engineeringtoolbox.com/nominal-wall-thickness-pipe-d_1337.html

If the inlet line is SCH 40 or SCH 80, everything is fine. If the inlet line is SCH 160 or XXS, you will need a new, bigger vessel nozzle.

Dejan IVANOVIC
Process Engineer, MSChE

 

[chemks2012]

e43u8 and Dejan

Thank you both of you.

Dejan,

It makes what you said I.e. Actual origins area and nozzle internal area should be compared. Is that given in API specifically?

e43u8

Agree what you said about 3% rule but don't you think Dejan is right in saying as long as the actual orifice area of the valve is equal or smaller than the vessel nozzle itself, it should be ok?

 

[EmmanuelTop]

You know what API 520/II says. It has been quoted in this thread. Other resources have also been referenced, and they introduce logically sound, engineering methods which allow the designer to consider special cases while not violating API 520.

To me it seems like you are expecting from us to find a special Code for you, in which explicitly says "Yes, if you have a 3/4" nozzle on a vessel and you have a 1.5" inlet nozzle of a PSV with F orifice, yes, you can connect this nozzle to the PSV and you are protected by this sentence from everything in the world". This code specially written for your case does not exist.

You are an engineer. You have to use your engineering judgement, supported by accurate calculations. If you can prove that there are no issues which will affect functionality of this PSV and the PSV will work as intended, then what is the problem?

Dejan IVANOVIC
Process Engineer, MSChE

 

[chemks2012]

Dejan,

I am just trying to understand and making sure if my interpretation of API and input from you all is correct! That's all.

I was bit confused reading some of the posts in this thread.

Thanks for your help once again.

KS

 

[georgeverghese]

One more thing that needs to be checked :
Presume your piping engineer and vessels engineer are aware of your intention to use this nozzle for a F orifice PSV - have they given you feedback on the suitability of this nozzle to withstand worst case nozzle loads during relief ?

 

[chemks2012]

georgevergese

Thanks. I believe you are talking about reaction forces. If so, will do.
If not, please elaborate.

KS

 

[georgeverghese]

Yes, it may be useful to see how they derive the reaction forces, since rapid open / close cycles can create large mechanical reactions on this small 3/4inch nozzle.

 

[EmmanuelTop]

I think you have two options basically, and everything boils down to these:

1) Either you consider the existing vessel nozzle as an integral part of the PSV inlet piping, which automatically means the nozzle is of insufficient size and you have to find another solution (drill another nozzle on the vessel or try to swap the function between the 3/4" and another existing 2" nozzle - if anyway possible), or

2) You don't consider the existing nozzle as an integral part of the PSV inlet piping, based on the referenced article in my post on 18th April, and you install 3/4" x 1.5" reducer on the nozzle. Then you perform all the required calculations (pressure drop, vibration, loads) and make sure that this solution is 100% functional (there is nothing that can further restrict the effective flow area towards the PSV).

In my opinion, the API 520/II does not mention vessel nozzle explicitly, so my approach would be to verify if solution (2) can be safely applied. But this is your call.

Dejan IVANOVIC
Process Engineer, MSChE