7 replies to this topic

[ogpprocessing]

Dear CHE Experts,

 

For a compressor station with centrifugal type compressor in which cases we may need to go for pressurized shutdown of the compressors? For this case I suppose suction and discharge ESD valves should be closed and compressor station is at its settle out conditions. Correct me if I am wrong.

 

Could you shed some light that:

 

1. Usually the seal gas source is taken from compressor discharge. If compressor is held at settle out condition then how the seal gas should be supplied? should we think about external source of seal gas?

 

2. During a recent Hazop meeting, one recommendation is raised to design compressor with the capability of start-up from settle out pressure. After negotiation with compressor vendor, they believe that by implementing this recommendation they have to select a more powerful compressor motor with higher rated power. Instead, they have proposed to perform partial depressurization of the compressor initiating from settle out pressure down to compressor suction pressure and then start-up the compressor from suction pressure.

I am not sure how much time saving we can gain if our compressor can be started from settle out pressure in comparison with suction pressure or zero pressure? Based on your experience is this common design for compressor motor to have this capability of start up from settle out pressure after keeping the compressor at pressurized shutdown condition?

 

3. Is there limitation of start-up of the compressor per hour? because there may be concerns for motor overheating upon several start up of the compressor.

 

4. Based on your experience how long the compressor station pressurized shutdown at settle out pressure can sustain? If this is too short such that we can not resolve the problem and subsequent depressurization is unavoidable then I do not see too much benefit with provision of this capability. What is your idea?

 

 

5. If the compressor is at pressurized shutdown state at settle out conditions, what will happen to u/s and d/s units? do we need dump valve to flare at suction to continuously route the gas to flare? How about d/s units? for example if we have gas dehydration unit at d/s then what will happen if u/s compressor is at pressurized shutdown state?

 

In my case the process line up is as follows:

 

Oil production unit => Gas gathering compressors => Sales gas compressors => TEG gas dehydration Unit => Fuel gas station, lift gas compressors and export gas pipeline.

 

6. If we want to re-start the compression train with pressurized casing (I suppose suction and discharge ESDV is closed) then what is the sequence of start-up? for suction ESDV the pressure is lower because compressor is at settle out pressure. So when to open the suction ESDV and how to overcome this higher pressure? 

 

 

[Bobby Strain]

You must provide a motor with sufficient torque to start at whatever pressure you choose. And, if you choose to start at settle-out pressure, the motor will require a higher current to start. So you need to check your power supply capabilities, too.

 

Bobby

Edited by Bobby Strain, 02 April 2013 - 07:51 PM.

[prengr]

Dear sir,

In my experience(my compressor had not huge capacity and high differential pressure), when the compressor re-starts at the settle-out condition, it was depressured using a depressurizing valve on the discharge line.

In my opinion, if your compressor is not huge and the period of shut-down is not so long or you have to re-start quickly, depressurization is  useful except loss of fluid in the compressor system.

And i think if you or the engineer shared this settle-out pressure with the compressor vendor during the the purchasing/design stage, this compressor might be considerd to re-start at the settle-out condition. If this compressor system was designed for settle-out condition, you can re-start directly.

Also, there is limitation of star-up of the compressor per hour that is usually 30 minute ~ 1 hour, as i know this limitation is provided by compressor vendor(motor vendor). You can find it out at the compressor operation manual or control narrative...

 

Thanks.

Edited by prengr, 02 April 2013 - 11:29 PM.

[Erwin APRIANDI]

Thread from previous dicussion

 

http://www.cheresour...rom-settle-out/

 

And trying to answer some of your question.

 

Could you shed some light that:

 

1. Usually the seal gas source is taken from compressor discharge. If compressor is held at settle out condition then how the seal gas should be supplied? should we think about external source of seal gas?

 

In all of my booster compression platform project an external seal gas using nitrogen bottles is provided

 

2. During a recent Hazop meeting, one recommendation is raised to design compressor with the capability of start-up from settle out pressure. After negotiation with compressor vendor, they believe that by implementing this recommendation they have to select a more powerful compressor motor with higher rated power. Instead, they have proposed to perform partial depressurization of the compressor initiating from settle out pressure down to compressor suction pressure and then start-up the compressor from suction pressure.

I am not sure how much time saving we can gain if our compressor can be started from settle out pressure in comparison with suction pressure or zero pressure? Based on your experience is this common design for compressor motor to have this capability of start up from settle out pressure after keeping the compressor at pressurized shutdown condition?

 

Time saving, I do not know we do not predict this during design most capable person who can answer the time saving is an operator and/or the compressor vendor.

 

Depend on the operator, if the operator allow a flaring, sometimes they will asked us to design for this, but if no flaring is allow or zero flaring policy mostly we just going to design it to start up at max suction pressure.

 

3. Is there limitation of start-up of the compressor per hour? because there may be concerns for motor overheating upon several start up of the compressor.

 

I have no idea about this, let others to answer. 

 

4. Based on your experience how long the compressor station pressurized shutdown at settle out pressure can sustain? If this is too short such that we can not resolve the problem and subsequent depressurization is unavoidable then I do not see too much benefit with provision of this capability. What is your idea?

 

I have no idea about this, let others to answer. 

 

5. If the compressor is at pressurized shutdown state at settle out conditions, what will happen to u/s and d/s units? do we need dump valve to flare at suction to continuously route the gas to flare? How about d/s units? for example if we have gas dehydration unit at d/s then what will happen if u/s compressor is at pressurized shutdown state?

 

Mostly what I know when discussing with operator of the plant during design and engineering stage, they will flare the gas and try to minimize the inlet wellfluid to the plant this is what happen in the upstream. While in the downstream if this compressor connected to the sales gas pipeline, they will just leave the shutdown valve close and let the gas flow till the pressure is no longer available to deliver the gas.

 

In my case the process line up is as follows:

 

Oil production unit => Gas gathering compressors => Sales gas compressors => TEG gas dehydration Unit => Fuel gas station, lift gas compressors and export gas pipeline.

 

6. If we want to re-start the compression train with pressurized casing (I suppose suction and discharge ESDV is closed) then what is the sequence of start-up? for suction ESDV the pressure is lower because compressor is at settle out pressure. So when to open the suction ESDV and how to overcome this higher pressure?

 

I have no idea about this, let others to answer. 

Edited by erwin.apriandi, 03 April 2013 - 12:43 AM.

[ogpprocessing]

Dear Erwin,

 

For the item no.1:

Sometimes the settle out pressure is high. For example in my current project settle out pressure of sales gas compressor is about 50 barg and for gas lift compressor this is about 90 barg. So there may be difficulties to provide seal gas with nitrogen bottles within this high range of settle out pressure.

 

For item no.5:

The d/s facilities for sales gas compressor are export pipeline, fuel gas and lift gas compressor. Then if sales gas is kept at pressurized shutdown:

 

For export gas pipeline the story is somehow similar to what you mentioned.

 

Fuel gas will be lost which is very critical for plant. The large users of this plant are dual fuel.

 

Lift gas compressor will operate at recycke mode. So lift gas will be lost and heavy crude oil production will be lost.

[Erwin APRIANDI]

Hi

 

"Gas can be stored in high-pressure cylinders ranging to 6000 psig (410 bar), normal-pressure cylinders ranging between 2000 and 2500 psig (140 and 175 bar) and low-pressure cylinders ranging up to 480 psig (34 bar)."

 

I qoute this from http://www.engineeri...lume-d_843.html

 

But as I experienced, this is right. Since I have calculate the amount of nitrogen required for purging from a 200barg Nitrogen Bottles, so 90 barg is considerably low.

[thorium90]

To answer no 3, the following links might be helpful.

 

http://www05.abb.com... GB 02_2005.pdf

 

http://www.bradleysm...& Intervals.pdf

[trinhduchanh]

Dear ogpprocessing,

 

I has few advice at below, please take look

 

1. Usually the seal gas source is taken from compressor discharge. If compressor is held at settle out condition then how the seal gas should be supplied? should we think about external source of seal gas?

 

If 1st stage suction pressure less than either nitrogent or fuel gas pressure available at central processing platform then they shall utilize these gas for dry gas seal console during start up sequence so that difference pressure maintain at 3 bars. If in case settle out pressure occurred then UCP will depressurize system compressor until equal with 1st suction pressure and simultaneously Open Nitrogent or Fuel gas SDV to maintain difference pressure with casing pressure for avoid the gas leaking through dry gas seal.

 

If 1st stage suction pressure bigger than nitrogent or fuel gas then they shall design one more gas booster pump inside dry gas seal console for build up pressure above casing pressure around 3 bar during start up and full blowdown or partial blowdown condition.

 

2. During a recent Hazop meeting, one recommendation is raised to design compressor with the capability of start-up from settle out pressure. After negotiation with compressor vendor, they believe that by implementing this recommendation they have to select a more powerful compressor motor with higher rated power. Instead, they have proposed to perform partial depressurization of the compressor initiating from settle out pressure down to compressor suction pressure and then start-up the compressor from suction pressure.

I am not sure how much time saving we can gain if our compressor can be started from settle out pressure in comparison with suction pressure or zero pressure? Based on your experience is this common design for compressor motor to have this capability of start up from settle out pressure after keeping the compressor at pressurized shutdown condition?

 

You are right. Vendor didn't agree for your proposal to start compressor from sette out condition. This must require more power consumption during start up. That why it never implement in practice. I don't feel that it would save more time than running from 1st suction pressure. As my experience, the timing for staring compressor from suction pressure it take around haft hours from starting to On Load. If your compressor equip with VFD this can ramp up motor at full load condition. 

 

3. Is there limitation of start-up of the compressor per hour? because there may be concerns for motor overheating upon several start up of the compressor.

 

Up to your application. If you use fixed motor to driven compressor then it will be limited by starting times due to motor overheating. Normally protection relay on MW swichgear shall be limit at 3 times/ hour. 

If your compresor utilize VFD for control the speed then also you can staring at any time by VFD. No more any staring times limit. That is one advantage of VFD versus fixed motor.

 

4. Based on your experience how long the compressor station pressurized shutdown at settle out pressure can sustain? If this is too short such that we can not resolve the problem and subsequent depressurization is unavoidable then I do not see too much benefit with provision of this capability. What is your idea?

 

During detail design stage, engineering department must calcualte sette out pressure for each stage. Base on this the protection setting for inlet scrubber such as High High pressure trip and PSV will be establish. With almost project we did arealdy then it can withstand long times. During surge test at field we try to simulate actual surge point and see response while antisurge valve fully open. At this time, settle out pressure will establish from discharge to suction.

 

5. If the compressor is at pressurized shutdown state at settle out conditions, what will happen to u/s and d/s units? do we need dump valve to flare at suction to continuously route the gas to flare? How about d/s units? for example if we have gas dehydration unit at d/s then what will happen if u/s compressor is at pressurized shutdown state?

 

In my case the process line up is as follows:

 

If compressor shutdown at pressurize condition then all SDV at suction and discharge of compressor will be Close for isolation with upstream and downstream process. It without harming with upstream and downstream. I don;t have your P&ID hence i can't fully understanding system but normally at gas come to 1st suction compressor always take from HP separator. In this case if Inlet compressor SDV close then PCV on HP separator will open gas to flare for maintain pressure at HP separator. 

 

Oil production unit => Gas gathering compressors => Sales gas compressors => TEG gas dehydration Unit => Fuel gas station, lift gas compressors and export gas pipeline.

 

6. If we want to re-start the compression train with pressurized casing (I suppose suction and discharge ESDV is closed) then what is the sequence of start-up? for suction ESDV the pressure is lower because compressor is at settle out pressure. So when to open the suction ESDV and how to overcome this higher pressure? 

 

As early explain, if you want to restart compressor from settout out condition then UCP will control BDV to open for relief pressure inside casing to equal with suction pressure condition. After casing pressure satisfied then automaticly UCP wiil send star command.

 then