5 replies to this topic

[shvet]

Hi, forum

 

As a contractor we are in charge of detailed design of a HC vacuum tower revamp. Licensor specified to replace the bottom pump with a new one, but unfortunately pump was purchased with more capacity than required and during some cases we have capacity less than min stable flow. We can do nothing with this pump and shall apply spillback to pump suction - to tower bottom. And we can't make appreciable changes in tower design. We have a problem - from one hand we shall make a spillback, on other hand we shall not make big changes in tower design or we face problems with licensor. Can anyone helps us? Any worth sources will be appreciated.

 

General figures:

- liquid - gasoil

- pump suction flowrate (normal) - 90 m3/h

- pump suction flowrate (pump best efficiency point) - 196 m3/h

- pump min cont stable flow - 40 m3/h

 

Q1: How can we put pumped liquid back to bottom in so way that:
- spillback doesn't produce many bubbles and these bubbles aren't carried by liquid flow to pump suction nozzle and cause a cavitation
- spillback isn't carried to liquid outlet nozzle and heated liquid isn't sucked straight back to pump
We can't put a big distributor to tower bottom since it can interrupt bottom tray to work properly. Anyway licensor wouldn't be happy to see any changes but the less changes we make the better. The best it would be just another one new nozzle in the tower bottom.

 

Q2: (concerned not only this case)
Are there any recommendation/practices/rules etc. for vessel nozzle diameter? Any type of vessel:
- towers
- separators
- storage tanks/vessels
- buffers/depulsators/recievers etc.
Any type of fluid and nozzles:
- liquid/vapor inlet
- liquid/vapor outlet
- two phase inlet

 

Any advice or opinion would be appreciated - I understood in general forum members are experienced in refinery/chemical processing. But please note that potentially we already have a problem with licensor and any competent sources would be extremely worth in this case. Anything we could use in correspondence.

Attached Files

•  tower sketch.pdf   239.5KB   30 downloads

Edited by shvet, 14 January 2017 - 02:01 AM.

[pavanayi]

Shvet,

This isn't probably the answer you are looking for, but just a thought from a different angle:

 

From your numbers, just performing a rough back-of-the-envelope calculation (assuming a developed head of 150m), the difference in power consumption by both the pumps is about 50 kW. At an assumed average power price of $150/MWh, the difference in the annual operating cost for the big pump is $65000 more than what it would have been with the smaller pump.

 

I know there are issues of lead times and project timelines and related problems, and my assumptions may not be valid, but I reckon cost of a new pump of 90 m3/h will be paid back within a year considering the difference in operating costs as well as the additional engineering required for the 196 m3/h pump (spill-back, new nozzle in the tower etc).

 

Maybe a proper cost benefit analysis is all it takes to get the proper pump and avoid all the potential issues faced by you.

[J_Leo]

This is an interesting topic.

 

Maybe it is better to replace the pump,  in the long run, this will save more on the operating cost. If you want to keep the pump, why don't you talk to the licenser to get their advice where to route the spillback?

 

I generally see the pump spillback goes back to the suction vessel. Is that possible for the spillback goes directly to the suction line? maybe this will carry gas to the pump? Has anyone seen this practically? I am just curious and want to learn.

Edited by J_Leo, 17 January 2017 - 12:16 PM.

[shvet]

 Maybe it is better to replace the pump,  in the long run, this will save more on the operating cost. 

 

We can't buy a new pump.

 

 

 If you want to keep the pump, why don't you talk to the licenser to get their advice where to route the spillback?

We intend to do this way.

 

 

I generally see the pump spillback goes back to the suction vessel. Is that possible for the spillback goes directly to the suction line? 

It leads to liquid overheating

 

 

 Has anyone seen this practically?

It is unpractical because of liquid overheating.

[J_Leo]

Shvet,

 

Thank you. I think your answer to my question on the spillback sounds reasonable to me.

[Dazzler]

Hi Shvet, 

Regarding Q2 nozzle sizing, you can web search for Line Sizing or Nozzle Sizing Process Design Criteria.   The velocity or other criteria depends on the service and phase and location and sometimes other factors.  Some locations such as pump suction lines have a number of criteria to check, including NPSH for pump.

Dazzler