8 replies to this topic

[Prashant Process]

There is a rotary air compressor capacity 325 scfm @ 32 deg C and Pressure 8.5 barg. There is a heat exchanger which maintains the outlet temperature ~ 32 deg C. The issue is that a PSV has to be sized for the air receiver. The set pressure of the PSV is 11 barg and relieving pressure 12.2 barg. The issue is if I use isentropic conditions it predicts relieving temperature of 62 deg C which would be true if there is heat input in the system. Should the relieving temperature be considered as 32 deg C. How to arrive at the relieving temperature. Advance thanks for your valued replies to this post.

[sheiko]

The issue is if I use isentropic conditions it predicts relieving temperature of 62 deg C which would be true if there is heat input in the system.

Could you please elaborate this statement?
This might help clarifying the issue.

Edited by sheiko, 23 February 2012 - 07:47 PM.

[Prashant Process]

Sheiko,

T2 = T1 * (P2/P1)^ (Y-1/Y) where Y is Cp/Cv

If Y=1.4 (for isentropic compression) then the T2 approx. 62 deg C theoretically.

This is true if there is external heat supplied to bring about compression.

For a blocked outlet case the pressure increase is due to the additional air introduced into the air receiver. Hence the temperature increase is not due to compression of constant air mass.

Please let me know if you require additional information.

[sheiko]

T2 = T1 * (P2/P1)^ (Y-1/Y) where Y is Cp/Cv
If Y=1.4 (for isentropic compression)

I believe the above mentionned formula is in fact the adiabatic compression equation, and that is what I would use as the relieving temperature in the gas blocked outlet case.
By the way, the correct maths are T2 = T1 * (P2/P1)^ (1-1/Y), with:
Y: ratio of specific heats (or adiabatic exponent, isentropic exponent, k-value).

Edited by sheiko, 26 February 2012 - 08:18 PM.

[mohds23]

The issue is if I use isentropic conditions it predicts relieving temperature of 62 deg C which would be true if there is heat input in the system. Should the relieving temperature be considered as 32 deg C. How to arrive at the relieving temperature.

Relieving temperature is dependent on relieving case i.e fire case or blocked outlet. You need to check whether your instrument air reciever falls within any potential fire zone or not.
Agreed with Sheiko's statement.

regards,
Sharique

[Prashant Process]

Sheiko,

The formula is for Isentropic compression please see the NASA link http://www.grc.nasa....ne/compexp.html

This formula indicates that the temperature of air in the air receiver will increase from 32 deg C to 62 deg C but this is not observed in field. This can be ratified from the temperature trends when the compressor is unloading. I require to know if the heat exchanger upstream of the air receiver provides air which is at 32 deg C and if pressure were to rise in the air receiver only by the virtue of blocked outlet, the above formula does not account of cooling action of the heat exchanger in-line. Can I assume the releiving temperature with some other relation.

[S.AHMAD]

1. How do you get 62^oC? The temperature and pressure in the adiabatic/isentropic equation is in absolute terms.
2. I presumed that the receiver is downstream of a cooler which is cooled the compressed air to 32^oC.
3. For sizing the PSV, the temperature shall be based on the emergency scenario of interest
4. For blocked discharge, with utility intact, then the temperature shall be 32^oC unless you are considering a double jeopardy where blocked discharge simultaneously with utility failure.
5. Based on my imagination of your P&ID, utility failure and fire cases are no credible.
6. Therefore, for sizing PSV, HX outlet temperature (what ever it shall be - consider fouling?).
7. However, for mechanical design of the receiver, the maximum possible temperature shall be used namely failure of heat exchanger.
8. Please check back you temperature calculation. Based on my estimate, you need multistage compressor with inter-coolers.

Edited by S.AHMAD, 01 March 2012 - 05:14 AM.

[Prashant Process]

Thank You Mr Ahmad for the detailed analysis.

1. How do you get 62oC? The temperature and pressure in the adiabatic/isentropic equation is in absolute terms.
T2 = 305 * (13.2/9.5) ^ (0.4/1.4)=335 deg K = 62 deg C

2. I presumed that the receiver is downstream of a cooler which is cooled the compressed air to 32oC.
Assumption is correct

3. For sizing the PSV, the temperature shall be based on the emergency scenario of interest
Correct

4. For blocked discharge, with utility intact, then the temperature shall be 32oC unless you are considering a double jeopardy where blocked discharge simultaneously with utility failure.
There is a temperature rise of 2 to 3 deg C observed when the pressure rises from 9.5 bara to 13.2 bara in field but I don't know about any correlation which can determine the same.

5. Based on my imagination of your P&ID, utility failure and fire cases are no credible.
Correct and we will not base the relieving temperature on that. We want to support our assumption of relieving temperature around 35 deg C on some correlation.

6. Therefore, for sizing PSV, HX outlet temperature (what ever it shall be - consider fouling?).

There will be a marginal change in relieving temperature.

7. However, for mechanical design of the receiver, the maximum possible temperature shall be used namely failure of heat exchanger.
The receiver is protected with a high temperature trip.

8. Please check back you temperature calculation. Based on my estimate, you need multistage compressor with inter-coolers. See 1.

[S.AHMAD]

1. My initial understanding is that the compressor sucks atmospheric air at 0 barG. Apparently the suction pressure to the compressor is 8.5 barG (from the calculation).
2. Is there any significant impact on the design between 62C and 35C? I do not think any significant change. The PSV size increases with temperature namely size for 62C is expected bigger than that of 35C. However, we always select the next higher size to ensure that the PSV meets some variation in operating conditions.
3. The same is also true for mechanical design of the PSV and vessel. For 12 barG operating pressure, 150# flange rating can accommodate up to 250C temperature.