Estimate Flow Rate to DA 4

[ChEMatt]

I have a faulty pressure regulator that was not designed for the service it is in and I must replace it. I was requested to find a proper control valve, rather than a regulator.

The regulator was to maintain pressure on a DA. The DA is fed by a condensate drum, going from 20 psig to 5 psig. The DA also has makeup water from an RO system.

How do I estimate the flow needed to maintain pressure on the tank so I can size a replacement pressure control valve?

Thank you for your help!

Onwards,

Matt

 

[rutherford703]

I guess DA representing deaerator. You have two water sources to the DA. The makeup water from the RO will have no effect on the pressure of the DA unless the makeup water temperature is high. Then, the only stream affecting the DA pressure is the water from the condensate drum. Simply heat and mass balance should be enough to find the flow rate of condensate to the DA.

Parameters should include steam flowrate and pressure, cooling water flow rate and temperatures (inlet and outlet temperatures).

 

[SeanB]

I am not sure I understand your question. Is the pressure regulator on the condensate tank or on the vent steam from the DA?

 

[ChEMatt]

rutherford, Sean, thank you for your kind replies.

The condensate coming back has no flow measurement, and is at temperature. The RO water, likewise, has no flow measurement and comes in cold/ambient. I can estimate the flows based on the CVs of the level controllers, and I do have a flow meter on the line to the boiler, so perhaps I can get a balance on the drum.


The pressure regulator is on a tee on the 300# steam line, which drops the pressure directly to 5 psig to the deaerator (DA).

Onwards,

Matt

 

[TD2K]

The DA takes the incoming water and heats it to saturation temperature. A small purge stream off the top of the deaerator is provided to remove the oxygen and other gases stripped out of the water as well as some steam.

You just need to do an ethalphy balance around the DA (H of the 30 psig steam and the h of the two water streams) to estimate how much steam is required to achieve this plus the purge rate.

 

[SeanB]

Yes the guys are right - you have to perform a mass and energy balance. Try to get your hands on the original design data for the DA if you can. Read a little about DA operation to understand the function and operation as well. One of the unknowns is the vent rate. You will probably have to assume a value. A good rule of thumb is 0.2% of the throughput. Let us know how you make out.

 

[hacksaw]

you size the pressure regulator valve for worse case conditions, i.e. maintain dearator pressure with maximum cold water makeup

so to hold 5 psig, you'll need steam enough to heat the maximum make-up flow to saturated liquid at 5 psig, convert the btu to #'s steam and you're done,

undersized regulators create a ton of problems

 

[steamdog]

If you don't know ANY flow rates you can make some estimates:
The boiler has a BTU or lb/hr value, this can tell you how much steam is being made which will be how much water the DA will put through.... EXCEPT you have to add blowdown, and in a perfect world the blowdown amount will equal the water make-up from the RO(some estimate it a 10-20% but this depends on water quality). of course you will not have this perfect world 100% condensate return.

Then you do the BTU calcs as mentioned above and don't forget the amount of steam for the vent.