20 replies to this topic

[ramlalithravi]

Hello,

Well i have some views about hot gas bypass valve provision in compressor system. As far as my knowledge, the purpose is to depressurize the system as fast as possible to prevent the compressor from surge than the antisurge control valve during start-up and shutdown condition.

My question, is it required only for Centrifugal compressor??, Where the system will encounter surge condition when compressor meets low flowrate throughput into the compressor. how about positive displacement compressor be handled incase of over pressure situation. I saw few examples of reciprocating compressor having recirculation valve ultimately to manitain positive pressure at suction of constant speed recip. compressor, additionally Hot Gas Bypass valve also required for Recip compressor?? if not required what will be the reason?

Also HGBV provided in centrifugal compressor system connected to suction drum,Consequently the downstream of the HGBV will be at hight temperature and pressure as it is a ON/OFF valve will have no pressure reduction across Bypass valve. How hot gas has been handlled in the compressor suction??

More importantly there is also bypass valve not considered for all the centrifugal compressor, what is the criteria for providing this facility? Because in some of the centrifugal compressor system are not provided with HGBV. what is the funda behind this to opt for HGBV.

[mav9rick]

Hi Ramesh,
I have encountered employed the HGBV few times now while designing centrifugal compression system. I will try to explain the reasoning here.

A recycle valve, more commonly known as antisurge valve has 2 functions.

Its primary function being ,prevention of surge in a centrifugal compressor. As you must be aware, a centrifugal compressor operates at very high RPMs. For this reason, the antisurge valve needs to be very quick in response. Sometimes, dual solenoids fited with quick exhausts are installed to ensure that the valve opens quickly when the operating point approaches surge line.

Antisurge valve also plays a secondary role of flow control /suction pressure control. A cebtrifugal compressor can be turned down in RPMs if it is being driven by a turbine or a VSD motor. However once the lower limit of turbdown is reached, further flow/pressure control is done by modulating the antisurge valve. Forexample, if you want the forward flow ot be less, teh control system will open the antisurge valve to recycle some of the gas back to the suction
Now the problem is that we have 2 flow criteria for sizing the antisurge valve. One governed by the antisurge flow and the other governed by the turndown requirement. In some cases, the turndown requierment yields large recycle valve sizes which in turn means longer modulation time between fully open and fully closed positions, which in turn, can jeopardise the antisurge prevention response. It is in these circumstances that the HGBV is employed to provide extra protection.

when the HGBV opens, the hot discharge side gas is dumped into the suction. However please not that the compressor is also shutdown at the same time so performance is not really a factor anymore. The compressor sealing system is usually designed to handle the hot gas anyway and therefore there is no concenrn as such.

I must admit, I have not seen HGBV on the recip compressor. More so because surging, which happenes because of internal backflow of the gas within a centrifugal compressor is not possible in a recip compressor. Also, recip compressor operate at much slower RPMs (~1000 RPMs) anyway so speed of response is not an issue.

I hope the above helped.
cheers

[ramlalithravi]

Thanks for ur precise explanation, understood the actual concept of why Hot Gas Bypass Valve required for centrifugal compressor system.

Still i would like to ask something which is not clear to me. U have mentioned, when the HGBV opens, the hot discharge side gas is dumped into the suction. However please not that the compressor is also shutdown at the same time so performance is not really a factor anymore.

Will HGBV function only during inadverdent condition while compressor went to shutdown??

If throughput to the compressor goes lower and falls on the surge line, in that case compressor will not operate further when HGBV intact to the system.Pls clarrify this point..

does it mean reciprocating compressor not require recirculation valve??

[mav9rick]

Ramesh,
I am not sure what your following sentence means:

"If throughput to the compressor goes lower and falls on the surge line, in that case compressor will not operate further when HGBV intact to the system.Pls clarrify this point.."
Opening of HGBV occurs during an emergency situation which threatens the mechanical intergrity of the machine. The compressor is also shutdown at the same time. You can appreciate that as the compressor winds down, the flow through is also falling. It is important to ensure that the suction is not starved of gas at that time else it will send the compressor into surge.
Usually, the surge is detected by vibration switches installed on the compressor pad. And yes, the HGBV will be toggled open whenever a trip is triggered by these switches.
The compressor power supply ( fuel gas/steam supply if turbine operated) will be cut off.

There is a difference between a recirculation valve and a HGBV. A reciprocating compressor can have recirculation valve but it is purely for control purposes as a recip doesnt suffer from surging phenomenon

[ramlalithravi]

I intent to ask about, whether the purpose of HGBV provision only during Emergency shutdown condition only or also to smoothen normal operating condition?. Because the advantage of providing Antisurge valve and HGBV are to prevent the system from surge and as if it will maintain flow throughput to the compressor always more than surge limit.

I came across some literature references, in which HGBV are used along with antisurge valve in order to maintain temperature at suction of the compressor, since antisurge valve outlet will have very low temperature and when it mix with HGBV gas could reach higher tmeperature at suction from operating point of view, hence we could not get more condensate in the suction drum.

And for more clarrification what is the need of recirculation valve in the reciprocating compressor??

[ramlalithravi]

I could find same topic discussion "HGBV need for Reciprocating Compressor?" but it was not ended up completely, pls give ur ideas..

[mav9rick]

HGBV is only used during emergency trips/shutdown and NEVER during normal operation. It is an ON/OFF valve and therefore it cant really perform a smoothning function anyway.

The idea is not to makecompressor recycle gas on a normal basis so as to avopid the surge. If you are having to do so in your percieved normal operation, then the compressor has been oversized. Under normal operation, the antisurge valve should remain closed. The pressure and flow is usually controlled by the speeding up or slowing down the compressor.

You may in some cases provide a small slip stream of discharge gas taken upstream of the aftercooler and mix it with the recycle gas to ensure that cold temperature dont exist downstream of the antisurge valve.This is for a completely different purpose and is not a safety critical function. Typically, low temperature trips and alarms are installed on the temperature controller downstream ofthe aftercooler so as not to overcool the discharge gas which will get further cooled due to the JT effect across the antisurge valve.

You can employ a similar bypass arrangement for reciprocating compressor as long as you take into account the slightly warmer suction temperature on recip compressor performance. The idea of HGBV there is not to prevent surge but to prevent hydrate formation or condensate dropout in the compressor suction system.

Recirculating valve is used in the reciprocating compressor to controll the flow or suction pressure of the compressor system. Recycle valve is opened to reduce the forward flow and increase suction pressure and closed to increase forward flow or reduce the suction pressure.

I hope teh above explanation is clearer.

[fallah]

Now the problem is that we have 2 flow criteria for sizing the antisurge valve. One governed by the antisurge flow and the other governed by the turndown requirement. In some cases, the turndown requierment yields large recycle valve sizes which in turn means longer modulation time between fully open and fully closed positions, which in turn, can jeopardise the antisurge prevention response. It is in these circumstances that the HGBV is employed to provide extra protection.

mav9rick,

As far as i know (and as you mentioned), hot gas by pass valve (HGBV) is an additional protection against surge during compressor turndown (or coastdown) that would be added (if any) based on dynamic simulation study on compressor.

Now, do you think HGBV would be opened just in the emergency shutdown (or start up?) and just for variable speed compressors?

Fallah

Edited by fallah, 16 October 2011 - 02:44 AM.

[ramlalithravi]

It is so clear ur explanation and appreciate ur prompt reply, but u did not regard anything on reciprocating compressor recirculation valve. what is the application of recirculation valve in Recip. compressor??

One more thing, i need to tell at this time If the centrifugal compressor is constant speed compressor with suction pressure control, it has to have certain minimum flow always by continuous recirculation, henceforth it would have recirculation flow even during normal condition. As it is a constant speed compressor it would posses only a characteristic curve, when flows down the operating point move towards surge point and consequently recirculation occurs even while normal condition also. I have experience in this kind of compressor (Constant speed cent. compressor) that's why i mentioned in my reply, recirculation could occur during normal condition.

[mav9rick]

Fallah,

The HGBV Ramesh mentioned in his first post is an ON/OFF valve only. This ON/OFF valve is not used during normal operation to push the compressor off its anti-surge line because the valve will remain open and will continue to dump hot gas to teh suction of teh compressor - a scenario which teh compressor is not designed for.

This valve is only opened in a pulse action (solenoid actuated) in addition to the antisurge valve opening (solenoid actuated) . Unless the slenoids are reset, both the valves will not go back to normal position. Therefore, the HGBV is always remains closed in normal operation and only participates in the safety function as and when called upon by the machine PLC.

Further along the discussion, Ramesh raised a question about the hot gas bypass to control the temperature downstream of the antisurge valve. This valve can be a normal control valve operating either on a temperature controller or if the flows are not deemed too high, it can also be an RO installed downstream. That particular valve and its function is completely different from the safety function performed by a solenoid actuated ON/OFF valve, which is generally referred to as HGBV.

[mav9rick]

Ramesh,
I agree that for a constant speed compressor, you may need to use recirculating valve to maintain a constant pressure. However generally on buigger machines it is not done because the it is essentially a waste of energy. For smaller machines, a control valve can be installed at the suction of the sentrifugal compressor maintain a constant pressure.

I have mentioned the purpose of the recycle valve for a reciprocating compressor in my earlier post. It is used for the same purpose - to control the suction pressure of the compressor. A recip compressor is often driven by engines with no or limited turndown capacity. You can unload the barrels but further turndown . if required, can be achived by recycling the flow.
Bear in mind that recycling under any circumstance and for any compressor type should be the last resort for controlling the parameters. Having said that, it is sometimes unavoidable because rotating machinery do not have large turndown capabilities

[fallah]

Fallah,

The HGBV Ramesh mentioned in his first post is an ON/OFF valve only. This ON/OFF valve is not used during normal operation to push the compressor off its anti-surge line because the valve will remain open and will continue to dump hot gas to teh suction of teh compressor - a scenario which teh compressor is not designed for.

This valve is only opened in a pulse action (solenoid actuated) in addition to the antisurge valve opening (solenoid actuated) . Unless the slenoids are reset, both the valves will not go back to normal position. Therefore, the HGBV is always remains closed in normal operation and only participates in the safety function as and when called upon by the machine PLC.

Further along the discussion, Ramesh raised a question about the hot gas bypass to control the temperature downstream of the antisurge valve. This valve can be a normal control valve operating either on a temperature controller or if the flows are not deemed too high, it can also be an RO installed downstream. That particular valve and its function is completely different from the safety function performed by a solenoid actuated ON/OFF valve, which is generally referred to as HGBV.

mav9rick,

And what about other part of my question: Opening of HGBV during start up and application of HGBV in fixed speed compressors?

Fallah

[mav9rick]

Fallah,
HGBV may or may not be toggled open to achieve a sweep thropugh the purging phase prior to the compressor pressurisation or start-up. Usally if there is an HGBV present, it is opened to achieve a sweep through the suction and discharge. The sweep gas is continuously vented to flare for a calcuylated periuod of time. After the time period has expired, the control system shuts the vent valve to flare and closes the HGBV and allows the compressor to pressurise though the suction system.

The application of HGBV, if deemed required, for a constant speed centrifugal compressor is no different to a variable speed compressor. However constant speed centrifs will not have a surge map/line. There will be a a fixed flow at which the machine will surge and therefore the sizing of the antisurge valve is a straighforward affair. However, I must admit, I have not come across a HGBV for a smal centrif thus far.

[fallah]

The application of HGBV, if deemed required, ...

mav9rick,

Which criteria dicatates need to HGBV for a compressor?

Fallah

[mav9rick]

There is not set criteria as such howevet the requirement is depermined during the dynamic simulation of the antisurge response phenomenon as I explained in my first post.
If a compressor is designed for large turndowns in flow and pressure, an antisurge valve will more than likely be backed up by the HGBV. For instance, in one of my projects, the compressor in question was acting as a front end field compressor handling very low pressure gas as one of its operating modes and also acting as a sales gas compressor in another independent operating mode. The machine had to be designed to cater for both operating modes. As you can appreciate, the suction pressure variation was quite large and we found that a single antisurge valve will be too big to allow flow control. Larger valve size meant longer actuation time between open and close positions.
We installed a 2" HGBV to short circuit the suction and discharge. Typically, such valve will require to go from fully closed to fully open position in under a second. This requirement may vary from vendor to vendor.

[fallah]

mav9rick,

Thanks a lot,

What were the set points of ASV and HGBV in your mentioned example?

Fallah

[ramlalithravi]

Yes numerical values will clarrify much better as being requested by Fallah.

[mav9rick]

Fallah,
A HGBV is triggered open by the PLC upon detection if a safety crtical trip requiring an emergency shutdown of the compressor or when manual emergency shutdown is activated. It therefore doesnt have a "setpoint" per say.

The ASV also didnt have a single setpoint for the antisurge function. The requirement was and typically is to maintain a 10% margin between the operating point and the antisurge for the current compressor speed.

As for flow and suction presdure setpoint, it varied from 25TJ for one mode to 180TJ for another. Pressure setpoint was 25 barg for one mode and 60 barg for another.

Hope this extensive discussion has answered Ramesh's original question

[fallah]

mav9rick,

As far as i studied, to protect compressor against surge multiple valve approach (ASV+HGBV) may needed when the volumes in either side of the compressor would be very large and based on this configuration there would be two operational scenarios at the first we have modulating actuation of ASV for minimum flow control and then if needed fast depressurization of compressor discharge by HGBV for shutdown due to lack of capability of ASV to protect lonely the compressor against surge. If so:

What is the role of turndown in compressor trip? If ASV not to be sized for turndown case (due to its size may become too large) can we suppose that HGBV would be opened to protect the compressor against turndown conditions and following that the compressor would be tripped? If ASV (even with large size) to be sized such that could cover the turndown case can we suppose there would be no need to HGBV and contrary to previous case compressor trip would be eliminated in tundown conditions?

Fallah

Edited by fallah, 16 October 2011 - 07:30 AM.

[ramlalithravi]

Turndown is nothing but allowable operating range for the centrifugal compressor between desgin point and surge line at any given speed, so i guess turndown flow rate for centrigfugal compressor same as surge flow. If not what is the criteria for fixing turndown for the compressor system.


@Fallah,
I should appreciate ur questions, in fact even my earlier doubts were also the same.


ramesh

[fallah]

Turndown is nothing but allowable operating range for the centrifugal compressor between desgin point and surge line at any given speed, so i guess turndown flow rate for centrigfugal compressor same as surge flow. If not what is the criteria for fixing turndown for the compressor system.

ramesh,

Turndown in centrifugal compressors is the possible mass flow reduction before the compressor would be encountered to surge phenomena and expressed as percentage:

100- 100*(surge point mass flow at design head/design mass flow)

Fallah