Hydrotest Clarification

[D.Poulios]

Hello Everyone,
I have a question regarding the Hydrotest procedure of some piping lines connected to a compressor. The compressor has 8 stages and the Suction design data of those stages are the following:

Stage 1: Temperature: 80 deg. C Pressure: 2 bar Hydrotest Pressure: 3 bar
Stage 2: Temperature: 80 deg. C Pressure: 8 bar Hydrotest Pressure: 12 bar
Stage 3: Temperature: 80 deg. C Pressure: 14 bar Hydrotest Pressure: 21 bar
Stage 4: Temperature: 80 deg. C Pressure: 32 bar Hydrotest Pressure: 48 bar
Stage 5: Temperature: 80 deg. C Pressure: 65 bar Hydrotest Pressure: 97.5 bar
.
.
.
Reaching all the way to
Stage 8: Temperature: 157 deg. C Pressure: 162 bar Hydrotest Pressure: 243 bar.

Now, the questions are the following:

1) Is the thickness of the piping defined only by the design pressure and temperature as ASME B31.3 suggests (304.1.2) or at some point the hydrotest pressure governs in the thickness calculation?
In the fifth stage for example I have a 14" pipe (SS316L) 1 mm C.A. with a defined thickness 14.27mm ( ASME B16.10). For this thickness the maximum allowable working pressure is *88.6 bar which is significant lower than my Hydrotest pressure.
Should I increase the thickness so the pipe can withstand the hydrotest pressure?

*To calculate the 88.6 bar I solved the ASME B31.3 (304.1.2) equation and found P.

2) This very high Hydrotest pressure (of again let's only mention 5th stage) also concerns me in regards to flanges. Based on the design data, and as per ASME B16.5 tables the flanges are selected to be Class 600** (material group 2.3).
For 600 rating the maximum allowable working pressure is 82.7 bar which is again significantly lower than 97.5 bar which is the hydrotest pressure. Should I consider 900 lbs flanges to withstand the hydro pressure or should I leave it as is considering that these flanges (600lbs) have been tested up until 1,5*P@38 degrees [bar]?

** ( Pmax@ 80 degrees = 74 bar > 65 bar = Pdesign. )

Thank you everyone in advance!!

 

[GBTorpenhow]

You do not need to increase wall thicknesses or flange ratings to accommodate hydrotesting.

For flanges the hydrotest limit is as stated in B16.5: 1.5x the ambient working pressure rounded up to the nearest 1 bar. Group 2.3 Class 600 flanges are rated for a hydrostatic test pressure of 125 bar.

For pipe, the design pressure compared to the max working pressure of the system is what matters. Hydrotest pressure will often exceed design pressures. Check the hydrotest pressures against yield and satisfy yourself that all is well.

In any case, B31.3 allows you to reduce test pressures such that stresses during the test are kept below yield or 1.5x component ratings at test pressure if needed.

 

[David3399]

B16.5 allows a hydrotest of 1.5x the 38Deg. C. value. See 2.6.
See also, 345.2.1. for piping hydrotest pressures.

 

[LittleInch]

That's some compressor with 8 stages.

your biggest issue which I can't understand is how these stages are connected and more importantly isolated so that when you power down how do you stop the whole compressor sitting at the end pressure and temperature?

You will clearly need multiple heat exchangers

But hydrotest pressure only governs if you've got a load of static head from one end to the other.

All piping and flange standards etc allow for the hydrotest pressure to exceed the MOP / Design pressure / MAWP. There is a formula also for how to modify (increase) the test pressure at ambient temperature to test properly pipes etc which have been designed to work at higher temperatures where the working stress is de-rated from the 38C rating. See 345.4.2 eqn 24

 

[goutam_freelance]

If the minimum thickness is as per code, there is no need to increase the thickness to take care of the hydrotest pressure. In fact, it is the other way round. You need to reduce the test pressure in case certain stress values under test conditions exceed the criteria given in Cl 345.2.1 (a) of B 31.1. However, please check the latest code.

 

[D.Poulios]

Guys you are absolutely right. This section of ASME B31.3 that says that you must compare the hoop stresses occurred during hydrotest to the yield strength, I had missed.

Thanks for the guidance.