Steam control valve sizing

[Sahkah]

Hello all ,
For a steam/50% glycol heat exchanger , the steam inlet to the heat exchanger is divided into two inlet lines : one line @1/3 of the total capacity and the other line is @ 2/3 of the total capacity . The total capacity of the heat exchanger is 10,531,300 btu/hr (10531.3 MBH) and the total steam flow rate is at 12129 lbs/hr (1/3 will have 4043 lbs/hr and the 2/3 will have 8086 lbs/hr), the steam inlet pressure is 125 psi. what would the appropriate control valve size and pipe dia for each of the lines (the 1/3 capacity pipe & valve and, the 2/3 valve and pipe size) ? Appreciate your feedback. Thank you

 

[DrRTU]

You will need a little more information on the glycol process to do a selection. I gather the small feed was selected as the fine temperature control to give the CV improved LWT control. I suggest you select your valve mfg rep and have them help with the application of body(s) and actuator(s). A little homework in advance of contacting the rep will help.

https://www.controlsouthern.com/wp-...sher-Control-Valve-handbook-fifth-edition.pdf

 

[mk3223]

Helpful for a better answer if sharing a P&ID-like sketch to show the clear process details, including the steam source size and pressure before and after the heat exchanger.
Per maximum steam flow requirements, you may select the steam lines size with the minimum pressure drop. And then, the control valve may be sized per required flow rates and up- & down-streams pipe size and pressure, etc. And, you may consult the vendor for the "actual" valve size based on these conditions.

 

[willard3]

Read here as there's no canned answer to this question

https://beta.spiraxsarco.com/Learn-about-steam

 

[Orsiz]

You know the inlet pressure and flow rates. The next step is to calculate the pressure drop, at the specified flow rates, in each segment taking into account al system components (pipe, fittings, steam trap, elevation, ...). The resulting pressure between the steam supply pressure and downstream pressure drop will be the control valve pressure drop. Consequently, this information can be used to calculate the valve coefficient.

Regarding the difference in flow, I would start by calculating the pressure drop in one line for a certain diameter (evaluate if the corresponding pressure drop is acceptable) up to directly downstream of the control valve. Now use this pressure to size the other line (and components) in order to correspond to the second specified flow rate. This will mean, obviously, additional resistance (smaller line diameter, ...) depending with which line you start the calculation (smaller or bigger flow rate).

 

[BronYrAur]

What is the pressure in the condensate line? If atmospheric, you will have significant flash. Plus, you will have a large pressure drop across the new control valves. May be difficult to control under load load, You may want to knock the pressure down with a PRV station before feeding the heat exchanger.

 

[DrRTU]

As others have stated, you need to define the inlet and outlet steam conditions. You only advised inlet pressure – superheated? At saturated entering and 14.7psia leaving, you can calculate a Cv of both control valves based upon a “critical” valve. Size main line piping for DP at the mass flow / velocity. Select the control valves based upon a critical Cv. I guess both valves would be numerous line sizes smaller than the main. Sequence between both valves may be complex and the pressure will have to be dropped upstream of the control valve(s). If your approach condition is saturated, most mfgs will want their modulating valves to be limited to about 20-50psig DP with a standard valve. This will push you into a PRV. I suggest you research critical and choked steam control valves.