22 replies to this topic

[S J]

Currently, Column bottom line is connected with heat exchanger shell side top nozzle , then, heat exchanger shell side bottom outlet is connected with pump suction.  Due to the routine through heat exchanger to pump, high pocket is inevitable. The problem is the elevation of highest pipe between column bottom and heat exchanger shell inlet. The elevation of highest pipe is higher than column bottom low level. If column level control is failed, high cavitation could happen at heat exchanger inlet.

Heat exchanger will cool down shell side fluid.

I'm wondering if column bottom fluid is enough subcooled through heat exchanger, generated cavities could disappear.

Please share anyinformation regarding pump suction high pocket and cooling.

[fallah]

S J,

 

Please submit a simple sketch of the system you described; it will be more helpful than a description to provide a proper response...

[S J]

Fallah

 

Please open attached excel sheet.

Attached Files

•  sketch.xlsx   17.07KB   51 downloads

[fallah]

 

Please open attached excel sheet.

 

S J,

 

Cannot open the file...would you upload it in PDF format...?
 

[breizh]

Hi ,

Similar issue with me.

Breizh

[S J]

Sketch is attached.

Attached Files

•  sketch2.pdf   115.05KB   52 downloads
•  sketch2.xlsx   11.36KB   24 downloads

[Bobby Strain]

Is this equipment in operation?

 

Bobby

[S J]

Hello, Bobby Strain

The pump is not yet operated.

[fallah]

Sketch is attached.

 

S J,

 

Please specify the vapor pressure of the MDEA solution at 129 C and 87 C.

 

Also specify if the column is equipped with dedicated LSL (level switch low) to trip the pump in low level at column bottom...

[S J]

fallah,

Pressure and temperature specified in excel sheet is from HMB.

Because stream is from column bottom, 129 C and 139 kPag for H2O + MDEA lean amine stream indicates saturation status.

In other words, vapor pressure at 129 C is 139 kPag.

For vapor pressure at 87 C, I don't know because simulation is not available for obtaining vapor pressure of amine solution.

I just estimate that 87 C and 39 kPag may be subcooled status of lean amine solution.

My concern is regarding siphon and cavitation at the highest point of piping during start-up and operation.

I'm not sure if cavities and bubbles could be collected in highest point of piping and some of cavities could be entrained into the pump.

Please share experience if anybody has experience or knowledge about this.

[fallah]

S J,

 

Actually, I did want to know about vapor pressure to check if the liquid at the pump suction is subcooled after passing through the exchanger.

 

Anyway, if the liquid inside the column is saturated and in no case the low liquid level would be lower than (or even equal to) suction line's high point, it's much less likely having bubbles at high point and for liquid level control failure, as I mentioned, you can consider a dedicated low level switch by which the pump triggered to be shut down...

[Hassan seif]

Fallah

even if the stripper column level dropped below suction line high point that wouldnt be haremful to the pump for two reasons 

1-the piping arrangement is going to work as a syphon pipe and there would be still some positive head on the pump suction nozzle

2-those pumps working with such service (transferring MDEA solution from stripper to absorber) normally will have low net positive suction head required - S J please confirm..

regards

[S J]

Hassan seif

For lean amine pump, NPSHa is 11m and NPSHr is 4.42m.

[shvet]

 

My concern is regarding siphon and cavitation at the highest point of piping during start-up and operation.

I'm not sure if cavities and bubbles could be collected in highest point of piping and some of cavities could be entrained into the pump.

 

 

According to design practicies in my country this kind of suction line layout is strictly prohibited. Siphon has a tendency to caught and collect bubles. The same practicies I've met in Exxon Mobil. Even though bubbles appear in not enough amount to cause 3% head drop (called NPSHR acc. to API 610) they can coollect and breakthrough to pump and cause impeller cavitation damage and intermittent head drop.

 

As per my experience in this case can help balance line, but there exist high probability it will not help. It is a way "to cath at a straw".

[fallah]

shvet,

 

I'm agree with you...

[S J]

shvet,

Do you mean 'balance line from highest pipe vent to column bottom' ?

Alternatively, how about highest vent connected with closed drain of atmospheric pressure ? 

[shvet]

 

Do you mean 'balance line from highest pipe vent to column bottom' ?

 

And yes and no.

 

Balance line from bubble trap. Bubble trap should be located in the end of siphon loop. As close to vertical downward line as possible. But please consider straight run margin for possible vortex formation. Actually there is no universal  "rule of thumb".

 

 

 

Alternatively, how about highest vent connected with closed drain of atmospheric pressure ? 

 

It dependes on difference between operating pressure of column and closed drain system / atmosphere and on possible pressure variations in column. It doesn't seem as good idea for me. There are so many ways to go wrong in this scheme and you add new way.

 

P.S.:

Remark - please consider excentric expansions (flat on bottom) for bubble trap in my sketch

[shvet]

Additional information.

 

In your sketch the frist "downward" siphon loop works as "sand trap". As per my experience there are always soilds in (rich) amine. Feedtray, downcomers and kettle reboiler always suffer from fouling, you are going to add one more "sand trap".

 

In your case suction line (should be) designed for liquid velocity <1m/s (0.6-0.8 m/s). It means solids can separate and accumulate in first "downward" loop. This solids will reduce bore area of pipe and rise liquid velocity. Velocity will rise up to transitional velocity (when solid-liquid mix is homogeneous). All it can cause pressure drop in suction line much higher then those calculated.

 

All it means NPSHA will reduce up to any of system limitation is reached.

[S J]

shvet,

Thank you for your comment.

Have you attached sketch ? File is not shown. If you uploaded some sketch, please upload it again.

 

And currently, as a result of rechecking piping route,

column low level is about 100 mm higher than high point of piping route.

But , it may still have danger of cavitation if friction loss is higher than operating level during operation.

 

I have one more question.

In case that column operating level is higher than the highest elevation of piping route + friction loss,

Is there no cavitation ? I mean, if operating pressure in the piping is a little (e.g 100mm) higher than vapor pressure,

can we say that " there is no cavitation in the pipe" ?

[breizh]

S.J ,

To answer to your question : Calculate   NPSH a and compare with  NPSH r , you need to have margin to prevent cavitation  (let say 0.5 m) .

 

Hope it's clear .

 

Breizh

Edited by breizh, 30 May 2016 - 12:38 AM.

[S J]

breizh

At pump suction, NPSHa is 11 m and NPSHr is only 4.42m due to cooling by heat exchanger at pump suction.

I'm wondering about cavitation in the piping located at the highest elevation before flowing to heat exchanger and pump.

In the highest piping, NPSH margin may be less than 0 ~ 0.1 m considering column low level operation.

I'm wondering , though operating pressure in the piping is still higher than vapor pressure, if operating pressure is too close to vapor pressure, then, how much cavitation could happen in the piping.

If ( column operating level + friction loss ) is lower than vapor pressure at the point of the highest piping,

cavitation will occur. In this case, I'm wondering if cavities generated in the highest piping will be removed and disappeared by cooling through heat exchanger before reaching pump.

Edited by S J, 30 May 2016 - 01:17 AM.

[shvet]

 

In case that column operating level is higher than the highest elevation of piping route + friction loss,

Is there no cavitation ? I mean, if operating pressure in the piping is a little (e.g 100mm) higher than vapor pressure,

can we say that " there is no cavitation in the pipe" ?

 

Simple sketch

[S J]

Thanks, shvet. Your sketch is much helpful to understand proper design for amine pump suction.