3 replies to this topic

[rha257]

Hello All,

 

I have two questions regarding the discharge valves of a multistage centrifugal compressor:

 

1. In the case in which a dynamic simulation shows that a hot gas bypass valve is required on the compressor discharge, I have usually seen two dissimilar check valves right after the hot gas bypass take-off line. However, in case a hot gas bypass valve is not required, do we still need to have the check valves at the discharge of the compressor even though we have the check valve downstream the anti surge valve take off line? If yes, why?

 

2. A compressor vendor once told me that, in case the discharge piping volume from the compressor discharge is too large, the anti surge valve size has to increase accordingly which will to a significant reduction in the backpressure in the recycle line during flow recycle operation which will impact the gas forward flow. I did not really understand this concept so if someone could please elaborate about this issue of increasing the ASV size due to increase discharge volume.

 

Thank you in advance

 

Regards,

Rami

[SawsanAli311]

These two dissimilar discharge check valves represent the point of segregation between your compressor stages in case you dont have an intermediate shutdown valve. The presence of these two non-slam check valves directly downstream and close to  the compressor discharge flange are also crucial in preventing reverse overspeed of the motor in case of sudden power failure and hence they close quite fast to prevent reverse inertial revolution on the compressor's shaft when the compressor goes into settle out conditions and force the equalization of pressure between the suction and discharge to be through the anti-surge recycle line rather than through the compressor itself. Also when you calculate the settle out conditions, the point of distinguishing your individual compression stages is also defined at the boundaries where either these two dissimilar check valves are there. 

 

For your second query.. always imagine that the larger the discharge volume which is seen by the anti-surge valve inlet.. the greater is the gas inertia and the larger is the effect on the compressor with respect to the flow reversal during surge i.e. larger gas interia, the longer is the surge cycle and greater is the damage. Hence, the anti-surge control valve size will have to be increased (which is to be confirmed by dynamic simulations). Roughly the sizing of the Cv to between ASV is 1.8 * Cv at the intersection with the Surge limit line at the minimum speed and 2.2* Cv calculated at the intersection of the surge limit line with the maximum speed. When the discharge volume is large, recommended Cv is 2.2*Cv as explained above. The sizing of the valve is performed by vendor and validated by dynamic simulation. Also take care for the momentum rho.v^2 and the associated ASV piping since a larger valve gives higher rho.v^2 which shall not exceed 200,000. Always keep in mind that an optimum design targets minimizing the volume between the compressor discharge flange, the discharge check valve and the inlet flange of the ASV in order to reduce the surge cycle time and the inertia of the gas at the discharge. 

[rha257]

These two dissimilar discharge check valves represent the point of segregation between your compressor stages in case you dont have an intermediate shutdown valve. The presence of these two non-slam check valves directly downstream and close to  the compressor discharge flange are also crucial in preventing reverse overspeed of the motor in case of sudden power failure and hence they close quite fast to prevent reverse inertial revolution on the compressor's shaft when the compressor goes into settle out conditions and force the equalization of pressure between the suction and discharge to be through the anti-surge recycle line rather than through the compressor itself. Also when you calculate the settle out conditions, the point of distinguishing your individual compression stages is also defined at the boundaries where either these two dissimilar check valves are there. 

 

For your second query.. always imagine that the larger the discharge volume which is seen by the anti-surge valve inlet.. the greater is the gas inertia and the larger is the effect on the compressor with respect to the flow reversal during surge i.e. larger gas interia, the longer is the surge cycle and greater is the damage. Hence, the anti-surge control valve size will have to be increased (which is to be confirmed by dynamic simulations). Roughly the sizing of the Cv to between ASV is 1.8 * Cv at the intersection with the Surge limit line at the minimum speed and 2.2* Cv calculated at the intersection of the surge limit line with the maximum speed. When the discharge volume is large, recommended Cv is 2.2*Cv as explained above. The sizing of the valve is performed by vendor and validated by dynamic simulation. Also take care for the momentum rho.v^2 and the associated ASV piping since a larger valve gives higher rho.v^2 which shall not exceed 200,000. Always keep in mind that an optimum design targets minimizing the volume between the compressor discharge flange, the discharge check valve and the inlet flange of the ASV in order to reduce the surge cycle time and the inertia of the gas at the discharge. 

Thank you for your response.

 

I had a couple of points I wanted to clarify:

 

1. What do you mean by "reverse overspeed of the motor"? Doe the motor go into reverse rotation upon power failure? Is that a normal phenomenon?

2. So my understanding is that these check valves serve as a boundary to limit the gas volume on the downstream of the discharge nozzle which will limit the likelihood of having a surge in the compressor during coast down. Is that correct?

3. Why do we require two valves on the discharge of the compressor while only having one valve downstream the ASV tap off? Is it because of the criticality of the position being right after the compressor discharge?

4. My understanding is that the check valves will not close until the SOP has been reached and the pressure has equalized on the upstream and the downstream of the check valves so how do they close straight away after the compressor shuts down?

5. I have heard that one of the valves is usually a non-slam check valve but I never really understood what is the reason of having to use this kind of check valve. Is it due to its higher reliability than a swing type or double plate type? Or is it because of it properties in reducing the shock waves on the upstream and downstream of the valve during closure?

 

With reference to the second point, what is this "momentum rho v^2" and what is this 200,000 valve represent? Also, I have heard that a larger ASV has control instability issues but what does that really mean? 

 

Sorry for all the questions but I am trying to understand these concepts and I tried reading multiple sources but failed to reach a proper consensus. 

 

Thank you for your time.

[SawsanAli311]

 

These two dissimilar discharge check valves represent the point of segregation between your compressor stages in case you dont have an intermediate shutdown valve. The presence of these two non-slam check valves directly downstream and close to  the compressor discharge flange are also crucial in preventing reverse overspeed of the motor in case of sudden power failure and hence they close quite fast to prevent reverse inertial revolution on the compressor's shaft when the compressor goes into settle out conditions and force the equalization of pressure between the suction and discharge to be through the anti-surge recycle line rather than through the compressor itself. Also when you calculate the settle out conditions, the point of distinguishing your individual compression stages is also defined at the boundaries where either these two dissimilar check valves are there. 

 

For your second query.. always imagine that the larger the discharge volume which is seen by the anti-surge valve inlet.. the greater is the gas inertia and the larger is the effect on the compressor with respect to the flow reversal during surge i.e. larger gas interia, the longer is the surge cycle and greater is the damage. Hence, the anti-surge control valve size will have to be increased (which is to be confirmed by dynamic simulations). Roughly the sizing of the Cv to between ASV is 1.8 * Cv at the intersection with the Surge limit line at the minimum speed and 2.2* Cv calculated at the intersection of the surge limit line with the maximum speed. When the discharge volume is large, recommended Cv is 2.2*Cv as explained above. The sizing of the valve is performed by vendor and validated by dynamic simulation. Also take care for the momentum rho.v^2 and the associated ASV piping since a larger valve gives higher rho.v^2 which shall not exceed 200,000. Always keep in mind that an optimum design targets minimizing the volume between the compressor discharge flange, the discharge check valve and the inlet flange of the ASV in order to reduce the surge cycle time and the inertia of the gas at the discharge. 

Thank you for your response.

 

I had a couple of points I wanted to clarify:

 

1. What do you mean by "reverse overspeed of the motor"? Doe the motor go into reverse rotation upon power failure? Is that a normal phenomenon?

2. So my understanding is that these check valves serve as a boundary to limit the gas volume on the downstream of the discharge nozzle which will limit the likelihood of having a surge in the compressor during coast down. Is that correct?

3. Why do we require two valves on the discharge of the compressor while only having one valve downstream the ASV tap off? Is it because of the criticality of the position being right after the compressor discharge?

4. My understanding is that the check valves will not close until the SOP has been reached and the pressure has equalized on the upstream and the downstream of the check valves so how do they close straight away after the compressor shuts down?

5. I have heard that one of the valves is usually a non-slam check valve but I never really understood what is the reason of having to use this kind of check valve. Is it due to its higher reliability than a swing type or double plate type? Or is it because of it properties in reducing the shock waves on the upstream and downstream of the valve during closure?

 

With reference to the second point, what is this "momentum rho v^2" and what is this 200,000 valve represent? Also, I have heard that a larger ASV has control instability issues but what does that really mean? 

 

Sorry for all the questions but I am trying to understand these concepts and I tried reading multiple sources but failed to reach a proper consensus. 

 

Thank you for your time.

 

Hi Rha 257, kindly find below point wise responses:

 

1. What do you mean by "reverse overspeed of the motor"? Doe the motor go into reverse rotation upon power failure? Is that a normal phenomenon? YES, we need to ensure equalizing of pressure doesn't occur through the compressor and rather is ensured to be within the anti-surge control line. 

2. So my understanding is that these check valves serve as a boundary to limit the gas volume on the downstream of the discharge nozzle which will limit the likelihood of having a surge in the compressor during coast down. Is that correct? Based on my understanding, we always require a check valve as close as possible to compressor discharge flange irrespective to whether there is a shutdown valve between the stages or two dissimilar check valve between the stages. So essentially, this check valve(s) serve in : what is explained in item 1 as well as minimizing the gas inertia and discharge volume affecting surge protection.

3. Why do we require two valves on the discharge of the compressor while only having one valve downstream the ASV tap off? Is it because of the criticality of the position being right after the compressor discharge?  Yes, it depends on the vendor's recommendations, I have seen designs which have non-slam dissimilar check valves right away downstream the compressor discharge flange as well as check valve downstream the anti-surge control tap off (which is after the two dissimilar check valves). 

4. My understanding is that the check valves will not close until the SOP has been reached and the pressure has equalized on the upstream and the downstream the aforementioned of the check valves so how do they close straight away after the compressor shuts down?

5. I have heard that one of the valves is usually a non-slam check valve but I never really understood what is the reason of having to use this kind of check valve. Is it due to its higher reliability than a swing type or double plate type? Or is it because of it properties in reducing the shock waves on the upstream and downstream of the valve during closure? Non-slam means fast closing / fast opening.. this means that when compressor suddenly stops it will be closing quite fast to enable fast equalization through the anti-surge line. 

 

With reference to the second point, what is this "momentum rho v^2" and what is this 200,000 valve represent? Also, I have heard that a larger ASV has control instability issues but what does that really mean?  The momentum criteria I mentioned is for sizing the ASV associated piping and which usually is impacted by the selected Cv of the ASV and is validated by dynamic simulations. For large Cv ASV tuning becomes difficult and  as you said will experience instability and poor contralibility.