6 replies to this topic

[ahyong]

Dear All,

I am a fresh process engineer with experience <1 year and currently I am dealing with pump hydraulic calculation for the FEED verification stage. This is my first time to do this and I hope you can kindly guide me and help me to solve my query. 

First, its regarding the design pressure of centrifugal pump. For my unit, there isn't any design pressure stated in the datasheet(suction,discharge), for the DP of  suction can I use the DP of suction vessel instead? How about the DP for discharge part? I've been referring to the process design criteria for pump but there isnt any information regarding DP besides the design margin for flowrate.

Second, I need to look for the min. static head and max.static head of the pump. However, The licensor does not state the LLL and HLL in their vessel drawing so I think we could assume the value according to design. Since the level instrument determines the LLL of the vessel, the instrument should be at least 150mm from the bottom TL, so can I assume that the LLL as 200mm from bottom TL? As for the HLL can I assume it min 0.3m below the inlet nozzle?

Third, some of the vessels required boots. If the outlet is coming from the boots, the min. static head is also started from the LLL of boots? Please correct me if I am wrong.

I have uploaded a simple drawing for your better understanding of my questions.

 

Best regards!  Questions regarding Vessel.xlsx   11.76KB   161 downloads

[chemproc]

The attached file might help.

 

P.S: In your question, there is no mention of NPSH, you might want to take that into consideration to avoid cavitation.

Attached Files

•  pump specification.pdf   202.97KB   329 downloads

[Hamid_r_2003]

1- Pump design pressure (shut-off pressure) in FEED stage to be calculated as following: (Design pressure of suction vessel-friction loss at suction line)+1.2*pump delta P at rated flow rate.

2- for min suction static head considere teh LLL at 150 mm from bottom tangent line to be in safe side.

3- you are right

[Dacs]

 Let me go with your question one at a time

First, its regarding the design pressure of centrifugal pump. For my
unit, there isn't any design pressure stated in the
datasheet(suction,discharge), for the DP of  suction can I use the DP of
suction vessel instead? How about the DP for discharge part? I've been
referring to the process design criteria for pump but there isnt any
information regarding DP besides the design margin for flowrate.

There's what we call a "shutoff pressure" for centrifugal pumps. It basically means the pressure developed by the pump at no flow. It's normally used as a basis for the design pressure of your pump manifold.

 

The design pressure for the suction manifold is a bit tricky. There's a portion of the manifold that follows the design pressure of the discharge (which is normally the nearest isolation valve from the pump suction side). Reason being is that segment can experience shutoff condition during pump operation.

Second, I need to look for the min. static head and max.static head of
the pump. However, The licensor does not state the LLL and HLL in their
vessel drawing so I think we could assume the value according to design.
Since the level instrument determines the LLL of the vessel, the
instrument should be at least 150mm from the bottom TL, so can I assume
that the LLL as 200mm from bottom TL? As for the HLL can I assume it min
0.3m below the inlet nozzle?

This differs with designers. Some might use LLL/LLLL and some might outright use the bottom tangent line/vessel bottom.

What I'll do in this case is to run multiple cases with different elevations and check its effect with NPSHa.

Third, some of the vessels required boots. If the outlet is coming from
the boots, the min. static head is also started from the LLL of boots?
Please correct me if I am wrong.

While what you said is the most severe case that can happen, you have to keep in mind that there's another liquid layer (light liquid) on top of boot and it may affect your calculation.

 

That said, I most probably would use the boot LLL for NPSHa calculation and the light liquid HLL for shutoff calculation.

 

Hope this helps

[ahyong]

Thank you for the reply and it does solve my problem!! Btw now I have another question and this time I uploaded a pump sketch.

By referring to the pump sketch, there are 3 streams for this drawing. The no.1 stream number is shown on the PFD where I can refer to the HMB data to get all the information (flowrate, pressure, temperature) etc. Let me know if there is other factor that could affect the data of stream no.1

My problem is on stream no.2 which comes out from the S/T Heat Exchanger. The data of stream no.2 also shown on the PFD which I can look from the HMB. At this situation of course i can easily read from the HMB table, but lets say if the stream no. 2 data are not provided, can I assume the pressure on that stream as the rated suction pressure of pump? How about the flowrate?

Now comes to the stream no.3 which there isn't any HMB data stated for that stream. In this case can I assume the pressure on stream no.3 as the discharge pressure of the pump? Can I assume also the temperature as the operating temperature of the pump? I also need to know how to find the flowrate for this. Looking forwards to your reply

 

Best regards,  Pump sketch.xlsx   10.74KB   155 downloads

[Dacs]

Looking at your sketch, you'd probably want to look at these:

1. Stream 1 properties should be taken from HMB

2. Stream 2 properties should also be taken from either HMB or the heat exchanger datasheet.

3. Stream 3 properties can be sourced from Stream 2 since there will be negligible change in values (since most properties are very weakly dependent on pressure but strongly dependent on temperature).

4. Pressures on PFD/HMB are just representative (very very rough estimation) when it was done. You have to carry out the proper hydraulics to obtain the pressure profile across the system.

5. Flowrate depends on HMB. Margin depends on your design basis/criteria

 

Just some heads up on what you're doing: if you have a saturated liquid as a source, you may encounter problems with your heat exchanger because it may be possible that with enough HEx elevation, you'll be having vapor formation in your suction system.

[ahyong]

Hi Dacs,

Thank you for your answer. For your information I am doing the rated case for the current calculation. Flowrate design margin is 10% acoording to instructions. I was told to look for flowrate from the pump datasheet which licensor provided. I've compared the flowrate (DS: 3347, HMB:3011kg/H). In this case should I take the highest one for the hydraulic calculation? Is it correct that the flowrate from stream 1 to stream 3 will be the same despite the temperature difference at 1: 48(HMB), 2: 40(HMB), 3: 40(Pump DS)? Besides that I also need to input the design pressure of the pump into the excel worksheet. By referring to the sketch can I take the design pressure of the source (vessel) as the design pressure?

 

Best Regards,