7 replies to this topic

[Bhavinkumar]

I am reviewing one vendor offer for HP Gas compressor. It is gas compressor used in gas processsing train upstream oil and gas Industry to boost the operating pressure of the gas. There are total 4 operating cases. Compressor is requried to design to increse the pressure from 1.05 MPag to 4.7 MPag (Refer Attached HP Compressor Datasheet for more Data). It is two stage compressor. Further please note that For alternate cases it is envisaged to use the same designed compressor to operate at 7.0 Mpag suction pressure to 4.7 MPag discharge pressure. Low suction pressure(e.g. 7.0 MPag) is due to a fact that after prolonged production from wells, the operating pressure of incoming fluid will decrease. Capacity required for comoressor is 1.6 MMSCMD for normal case. We asked vendor to design machine for 1.6 MMSCMD at 1.05 MPag suction pressure. And also asked vendor to give flow rate when operate at less suction pressure as above. 

 

We received the vendor offer (Refer attached Vendor Offer). As per vendor offer Compressor will deliver less flow rate during alternate operating case (low suction pressure case). Now here I have confusion. My limited knowledge about the reciprocating compressor is not makeing clerity on this point. As I know the compressors are constant volume machines. So here, when suction pressure is decresed means density is decreased. That means low mass will flow from the compressor resutling in low power consumption. But I am not able to understand how the capacity is reduced for low suction pressure case. But RPM of motor remains same. Does it mean the same energy from motor is delivered to the fluid ...then how come power is reduced.

 

Appriciate if you would throw some light in this case. I mean how compressor capacity will be reduced in this alternate case (low suction pressure case)

Attached Files

•  HP Compressor Datasheet.pdf   277.17KB   41 downloads
•  Vendor Offer.pdf   162.44KB   34 downloads

[PingPong]

For alternate cases it is envisaged to use the same designed compressor to operate at 7.0 Mpag suction pressure to 4.7 MPag discharge pressure. Low suction pressure(e.g. 7.0 MPag)

Note that you mean 0.7 MPag instead of 7.0 MPag.

 

But I am not able to understand how the capacity is reduced for low suction pressure case.

You are looking at the capacity in millions of Standard cubic feet, and obviously that will reduce as the suction pressure reduces.

 

At the same RPM a recip compresses the same Actual suction volumetric flow, but at lower inlet pressure that is equivalent to lower quantity of Standard cubic feet.

[breizh]

http://checalc.com/calculation.html

 

Hi Bavhinkumar ,

 

You may use the calculators attached to support your query .

 

Breizh

[Bhavinkumar]

@ Ping Pong

I regreat for my typo error. And would like to say that your understanding is correct. The pressure value in alternate case is 0.7 Mpag and not 7 Mpag.

 

1) From your reply I pick the punch line that for recip machine once its driver is designed it will always work at constant RPM regardless of change in process flow/suction pressure (Please correct me if my understanding is not correct). So this constant RPM will deliver in constant actual volumatric flow rate. The only difference would be may be change in power if suction pressure reduced.

 

2) Further, If we follow above understanding then I have one more question in my mind. It is related to LP compressor which is upstream of the HP compressor as mentioned above (Sorry for note attaching simplified PFD, but I am a new user so struggeling where to get attachment tab in response reply..... although I have made PFD). HP compressor have two sources of process gas one is from LP compressor and another one is from Inlet Separators. Operating Conditions of LP compresso is as follow (Normal Case):

 

Suction Pr - 0.23 Mpag

Discharge Pr - 1.05 Mpag  

Flow rate - 0.8 MMSCMD

 

During Alternate case discharge pressure required is 0.7 Mpag which is suction pressure of HP compresor and operating condition from Inlet Separators

 

(I have also LP compressor datasheet/Vendor offer but my inability of locating attachment tab is culprit)

 

We asked vendor offer for above normal case and ask vendor to advise the MMSCMD flow rate during alternate operation case (e.g. same suction pressure but at reduce discharge pressure 0.7 MPag).

 

As per vendor offer during alternate case, MMSCMD flow is decresed.

 

Now here as per the previous reply from Ping Pong, I am thinking that machine will deliver constant actual volumatric flow with constant RPM. Here, at suction side there is no change in pressure and hence density. However, discharge pressure is reduced. Then how MMSCMD is reduced for this case even thogh the constant condition at suction side.  

[PingPong]

You managed to attach documents in your first post, so I don't understand that you don't succeed to do it again.

 

Click on [More Reply Options] button at the bottom, and then at the bottom of the new screen under Attach Files click on [Choose Files ...] and finally on [Add Reply]

[Bhavinkumar]

Thanks Ping Pong

 

I have attached the LP Compressor Datasheet and Vendor offer and simplified PFD as well. Appriciate more light on subject matter.

Attached Files

•  LP Compressor Datasheet.pdf   274.74KB   13 downloads
•  Simple PFD.pdf   179.73KB   24 downloads
•  Vendor Offer (LP Comp).pdf   146.69KB   17 downloads

[xavio]

Bhavin,

 

I am not sure which MMSCMD is reduced in LP compressor case. Please check again.

 

Anyway, since you are operating at constant suction pressure, the MMSCMD should be more or less constant for all cases.

The minor difference you're concerned about is caused by suction temperature difference and volumetric efficiency effects.

When you decrease the discharge pressure to 0.7MPaG, you basically reduce compression ratio at each stage.

Different compression ratio produces different interstage density, temperature, etc.

Those factors in one and another way affect the overall flowrate of the compressor.

 

Yes, recip comp is a constant-volume machine, but it is actually not infinitely constant

 

Good luck.

 

xavio

[Bhavinkumar]

Thanks Xavio

 

Also thanks to everyone for their help and suggestions.