Question for Brad and Other Engineering types....Boilerpro

The Steam Whisperer (Formerly Boilerpro) · January 2009

I am looking at a steam system with two 3 inch HOneywell on/off valves and need to know their capacity. I spoke to Honeywell Tech and they found the formula of:

Pressure Drop = 20% (steam supply pressure - outlet pressure)

Is it really that simple?

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bob young · January 2009

would honeywell lie????

Steamhead (in transit) · January 2009

Do You Want the Valves

To modulate the flow or just be on-off?

As I understand it, there will be a pressure drop across a typical Honeywell globe valve based on it's Cv rating. What are the loads on each valve?

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Brad White_203 · January 2009

I am with Gordo

In general, for on-off control (not modulating control), the lowest pressure drop wins.

For modulating control, it is entirely load dependent and what you need for pressure on the outlet side of the valve.

Say you have a load of 500 PPH and have 5 PSIG in the line. Say the line size is 3" IPS.

If you use the 20% rule of thumb, you accept a 1 PSIG drop across that valve. A 2.5" valve will do that, but a 3" valve will impose about a third of a pound drop. I would go with the larger valve for on-off use. Save the cost of reducers but the valve may cost more. Not sure of the cost delta.

The 20 percent rule of thumb has been around a long time but like anything else, the specifics of your application must be known. In general terms though, I would not want more than that loss for an on-off valve. More to the point, that is about the most loss I would want to see in my entire system -for low pressure steam anyway.

The Steam Whisperer (Formerly Boilerpro) · January 2009

This is an ON/Off control

I am just starting to look this system over. This is two pipe and there are Dunham Bush traps on many of the rads...many are very large diameter. They have repalced a number of traps recently and the system operation has greatly improved, but fuel usage seems very high. It appears there are 2 -4inch mains feeding north and south, each with a zone valve. They are running 5 psi in the two 1.5 million input gas boilers and supposedly that's the only way they can heat the building. The valves appear to be rated at 100CV...so how does that convert to steam pressure drop? So what I can surmise from Brad is that 20% is the maximum target drop, not necesarily the drop on this system.

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Steamhead (in transit) · January 2009

Yikes!

If we suppose that only one boiler is needed and produces 1200lbs of steam per hr, and only one zone is open, and if we suppose a 4.5 psi loss across the valve, it looks like a Cv rating of 115 is needed.

That is using a formula from my Sarco book.

It looks like someone looked at the price of a 4" valve and convinced themselves that a 3" will do.

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The Steam Whisperer (Formerly Boilerpro) · January 2009

Thank you......

and what is the formula....Honeywell tech doesn't have it. That seems to jive with them having to run 5 to 6 psi at the boiler in order to get the system to work. I suppose I could just hook up a pressure gage on one of the radiators (rad vents had been installed)to see what the system pressure is after the valve.

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Jamie Hall · January 2009

There's another joker in this deck...

We're looking at low pressure saturated steam here. The problem is that with saturated steam, if there is any pressure drop there will also be some condensation. It is quite possible to get enough condensation because of the pressure drop across a valve with sat. steam to get surprisingly little steam past the valve, even when it is wide open and even when the usual tables suggest that you should only lose 'some'. You really do want to go with as big a valve as you can; full port if possible and, if it's just on/off, not throttling (which is another problem altogether) a gate valve rather than a globe valve, again if possible.

The Steam Whisperer (Formerly Boilerpro) · January 2009

I believe the valves are just earlier generation Honeywell ...

V5011's. Any suggestions on appropriate valves. The fuel bills are outrageous $5,500.00 in december with about 85 cents per therm nat gas.

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Steamhead (in transit) · January 2009

The Fomula is

Cv = W / 2.1(square root of delta P(P1 + P2)), where P1 = inlet pressure, P2 = outlet pressure, W = capacity lb/hr. This is for saturated steam, noncritical flow.

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The Steam Whisperer (Formerly Boilerpro) · January 2009

Thanks again.......

It looks like with more reasonable pressures of 3 psi in the boiler and 1 in the system, we need a CV of about 400...YIkes is right. I need to talk to the owner and see if he is willing to pay for some consulting and design work.

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Steamhead (in transit) · January 2009

Yes

I guess one of the bits of data you'll need is the true load. Let's hope they'll only need one of those boilers after you replace that valve. With what? Don't know yet!

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Steamhead (in transit) · January 2009

Another thing to look at

how does the air get out of the system?

Your situation sounds similar to one of our buildings that was maintaining 6 PSI 24/7 during the heating season. It has two zone valves that are actually smaller than the mains they serve. We straightened out the return piping and installed a huge vent pipe, and the boiler never has to exceed 1 PSI now.

http://forums.invision.net/Thread.cfm?CFApp=2&Thread_ID=59088&mc=13

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The Steam Whisperer (Formerly Boilerpro) · January 2009

I was looking at that too....

The 4 inch mains have 3/4 or 1 inch F&T at the ends, so they obviously need more venting. The rads are equipped with thermostatic traps, many of which have just been replaced. I need more data on the system to see if it was a vacumn system or not when installed, so I probalby shouldn't be putting in main vents yet till we know. This is a very high end commercial office structure. Marble walls in the hallways, polished brass accents and hardware....the works. I told the owner to turn the pressure down and just see what happens.

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Steamhead (in transit) · January 2009

See if you can find

the original plans for the building. These would probably show a vac pump if one was there originally. The school board might have them, since they had it built.......

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Question for Brad and Other Engineering types....Boilerpro

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