12 replies to this topic

[Prabhat123]

For one of our grass root vacuum distillation unit we have designed the vacuum column along with overhead ejector and condenser system. Ejector package including condenser is being provided by external package vendor. Cooling water supply pressure and cooling water return pressure (Both at ground level battery limit) to condenser (as provided to vendor) is 3.5 Kg/cm2(g) and 2.5 Kg/cm2 (g) respectively. As per the drawing and layout (Layout attached) provided by vendor (three stage ejector/condensers) cooling water supply pressure at inlet of first stage condensers appears to be around 0.34 Kg/cm2g considering static head and frictional loss. At the outlet of condenser pressure becomes   -0.16 Kg/cm2g considering differential pressure across first stage condenser in cooling water side as 0.5 Kg/cm2.

 

We are in apprehension that cooling water to condenser will not flow due to vacuum formation at the outlet. At present cooling water isolation valve at condenser elevation. Will placing the isolation valves at ground level help in water circulation across condenser?

 

Prabhat Choudhary

Attached Files

•  Ejector condenser elevation lay out.pdf   66.61KB   84 downloads

[shan]

No.  The valve location will not change the pressure drop across the valve.  The condenser outlet pressure -0.16 Kg/cm2 is OK, if it is still higher that the water vapor pressure at the temperature.

[Prabhat123]

Will there be any operational ,mechanical or safety problem arise, if butterfly valves in return line is shifted at ground level. We are worried about the support system of return line and butterfly valve due to possible vacuum formation.

[Bobby Strain]

If you want the system to work, you must add a booster pump to your cooling water supply. Shan's advice is not sound.

 

Bobby

[Prabhat123]

Thanks Bobby.

So you mean to say system will not work without booster pump? I will appreciate if yo can please elaborate in detail.

 

Thanks again.

[PingPong]

Is this a crude oil refinery VDU ?

 

Do you have a P&ID of the system?

 

How certain is that 3.5 kg/cm²(g) cooling water supply pressure?

 

Who designed the layout of the condensors with different elevations? Your company, or an engineering company, or the ejector package vendor?

 

Why is elevation of E-08 and E-09 so high?

 

What is the elevation of the hotwell vessel to which the condensate from the condensors drains?

 

It seems to me that a proper design could have all condensors at 23 m like E-10, with hotwell vessel at say 6 m elevation, hotwell diameter of say 2 m, and 15 m elevation difference between top of hotwell and bottom of condensors.

 

What is the status of the design, or the construction, of this system? Is this only a paper design sofar, that can still be changed without huge costs, or is it already under construction?

Edited by PingPong, 14 August 2015 - 03:19 AM.

[Prabhat123]

Thanks for the reply.

It may please be noted that ejector package has been provided through vendor as a package item and in fabrication stage at present. Also cooling water supply pressure is as per data provided by client only, so no further change can be done on it.

 

Hot well elevation from grade is 7.7 meter for NPSH required of the pumps below it.

 

I appreciate, if you comment on cooling water flow through condenser will establish or not?

 

Regards

[breizh]

Hi ,

May be good to read !

As pointed out , the question is really about the water supply (granted  Pressure  and flow rate) . Definitely a Booster pump is a safe option to be considered. .

 

Breizh

[PingPong]

Prabhat, thanks for not answering most of my questions. Here are a few more that you may choose to ignore:

 

ejector package has been provided through vendor as a package item and in fabrication stage at present.

Does that ejector vendor also provide the whole steel structure in which the package and the hotwell are installed?

 

cooling water supply pressure is as per data provided by client only

Provided by client? I thought you (your company) is the client. Or not?

 

I appreciate, if you comment on cooling water flow through condenser will establish or not?

In the absence of more detailed design data, I tend to agree with Bobby that you probably need a booster pump to assure proper operation.

The cooling water from the cooling tower is saturarted with air at 1 atm, so when the pressure drops below atm, air bubbles will form in the high points of your system and accumulate into an air pocket.

 

You should really ask the person responsible for the design of this system how this is supposed to work properly, in view of the expected negative gage pressure at the highest condensor. And ask him also how this system is to be completely filled with water at start-up.

Edited by PingPong, 14 August 2015 - 09:16 AM.

[katmar]

Consider the return leg pressure. If the battery limit pressure for the return water is 2.5 kg/cm²(g) and the top of the loop is 32 m above that then the pressure at the top of the loop is -0.7 kg/cm²(g) or 0.3 kg/cm²(a).  It could be argued that this is still above the boiling pressure for the return water, but that argument depends on the battery limit pressure being constant at 2.5 kg/cm²(g).  Any downward fluctuation in the battery limit pressure could result in boiling and vibration.  Many plants work this way, but I would not deliberately design it to be like that.

 

Whenever I have had to mount condensers or exchangers this high then I have always put a vent to atmosphere at the top of the loop, and I have designed the vertical section of the return leg to be self-venting.  This does result in larger cooling water pumps being required and more power being used, but it gives stable operation.  I suppose you could put pressure controllers on the cooling water loops to prevent boiling, but I have never seen this actually done.

 

You have not indicated who specified the heights of the condensers and the cooling water routes, so I cannot comment on the contractual situation.

[Prabhat123]

Thanks for your reply. I want to add that hotwell bottom elevation roughly of 7 meter has been fixed by client due to space constraint of the unit so as to put some small equipments under the hotwell area. Due to this condenser height appeared higher than typically normal value.

 

As you said water will flow acorss it by properly designing return leg for self venting but of course you wont deliberately design it.

 

Agreed sir, Can you clarify little more that vent shall remain open throughout? Placing of return leg butterfly valve at grade elevation will additionally help for water circulation or mechanically with respect to support and all due to vacuum formation upstream of return valve in case of valve is at elevation.

 

Thanks in advance.

[katmar]

Yes, the vent at the top of the cooling water return leg remains open during operation.  This ensures that a vacuum can never develop.  But because it is open to atmosphere there will be entrainment of air into the cooling water.  This air must be allowed to disengage before the water is returned to the cooling tower and this is why the vertical down leg in the return line must be made self venting.  If you decide not to install the vent then there is no requirement for the line to be self venting.

 

I have not thought through the valves completely, but if there is any vacuum formation on the return valve it will be on the downstream side rather than the updtream side.  It is difficult for me to get my head around the function of these valves because I have never used this type of closed sytem. I would probably position the valves for operator accessibilty, but you are correct that the pressures need to be checked as well.

[Prabhat123]

Thanks sir,