Regulating Pressure: 10,000 gallon tank with formed gas, 30 psig, 3" line to 1/4" ball val

[Subystud]

My system of interest is attached. This is a client's system I have been asked to analyze - I have never seen a gas handling system like this.

The Cv of the valve is 13, and the gas handled in a mixture of air, NOx, and CO2. This gas can be emitted atmospherically but must be done at such a velocity to

The gas is initially attempted to be scrubbed in a flooded line with reduced compounds. This flows from a 3" line to a 1/4" line. My initial impression was, "Okay, 3" is 144x the area of a 1/4" valve"

Question:
Is it possible to regulate the headspace gas a desired via 1/4" ball valves? I need to jet the material in the headspace.

 

[LittleInch]

Forgive me if this appears a little obvious, but from your description and actual tests, clearly a 1/4" valve combined with an unknown lenght of 1/4" S Stl tubing is unable to regulate the vessel pressure to the 15 psig required. Without some sort of gas production rate and lengths of 1/4" tubing which by the look of it someone has retrofitted, it isn't really posible to do any calcualtions to show what size you actually need, but 1/2" or 20mm would seem a better option. the 3" valve probably was a bit too big to do an an/off type actuated pressure control, but 1/4" looks way too small.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Subystud]

Update: process observed. Referencing initial post pdf.

The following occurs:
1. At process initiation, tank fills to 10K gal. The air (at atm) in the tank is compressed in the length of pipe upstream of check valve to a pressure of the head. Observed a steady rise in pressure to a final value of 5.8 psi in the gas at end of fill. Head was 12ft, works out. At 4k gals. pressure was neutral and all subsequent air compressed overcame flooded line.
2. At chemical dosing, process followed exactly as described by operator. Took 1 hr, but system stabilized at 26 psi on the gas, and 47 psi on the recirc pump discharge. Pump had 20 ft of head on it, 10 psi + 26 psi makes it 11 psi for pump discharge. Makes sense.
3. The pressure setpoints were inputs for when CV2 and CV3 opened. CV2 set at 13psi (dosing initiation +00:16:00), and CV3 at 15psi (+00:16:30). Both valves opened accordingly, and the pressure continued to rise to the steady conditions described.

It is still processing at time of typing, so relief not monitored.

With information gathered, I will be begin my assessment. The following is my baseline:
1. Control valves will be placed at ground level for ease of access.
2. Controls valves need indication /limit switches.
3. Control valve cycle time (eg ball valve) is a concern, the current ones take 7 sec to cycle. Going to 1/2" is 4x the area of the 1/4", and I do not want a "whoosh". Solenoids stick out here, but when considering costs you may a well use a modulating ball valve on a PID with the Pressure Indicator.

So what size Ball valve on a 4-20mA loop would be sufficiently large? Or how would you handle this gas?

Also operator noted pressure hit 35 psi and relief managed gas. I'd recommend a complete mechanical solution as well.

Thanks -

 

[LittleInch]

Your description above doesn't match the pdf which says only that CV2 opens. I had assumed CV3 was only for final venting. Hence this now sound like two 1/4" valves are open and you still have increased pressure so at least a 1/2" valve and possibly bigger is required. You don't mention what CV1 is doing or why you can't open that to relieve some of the pressure if you really want to keep it to 15 psig. If this is a pinch valve, why can't you modualate / control that?

"A modulating ball valve" sounds like a control valve to me. For this duty you probably need a 1" supply but to size it to do the duty, you really need to work out or estimate what your mass or volume flow rate is and then you can get a vendor to size it properly if you're going down the much more acccurate PID 4-20mA control loop. A normal ball valve does not make for good controlability.

There are a few odd ball things here - 7 seconds to turn a 1/4" ball valve from on to off??

Other alternative is to just add more 1/4" valves in parallel to CV2 with its own piping from the main header and gradually open them in turn as the pressure rises at 15, 16,17 ..... until you don't get a pressure rise above what you want.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Subystud]

Very true about CV3 for final venting. This is how the operators had it configured, and rightly so given 1 couldn't manage pressure, use 2. But they had it such that they were both venting to that pipe.

The 3" Pinch Valve, CV1, was the original equipment. The customer had specified it get used. It is the wrong design, IMO - called the company and they these are rarely used for gas handling applications.

Issues with 1/4" Ball valves (CV2 & 3) are as follows:
1. Cv value quoted is only for delta P > 300 psi.... Actual is 10-20psi
2. 7 sec cycle time is real - 120VAC powered actuator

1/4" Ball valves are spec'd are $400. 1" Non-differential pressure Solenoid with 10 ms cycle time is $150 with 316SS wetted parts. I will utilize this solenoid with a downstream Pressure Transmitter.

Thanks -

 

[LittleInch]

If your valve is full bore then your valve might not have a CV worth quoting. normally you only see it for control valves and for valves which are not fully open or reduced bore. Assume you meant upstream presusre transmitter for the 1" valve....

Pinch valve is an odd choice ok, - normally for granular / slurries and probably doesn't have enough controllability for gas.

You could also look at swapping that valve (CV1) out for a properly sized control valve?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way