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Steam Is A Small Volume

01/25/2012 2:57 PM

Hi all, first time here. I would like to know if there is a formula for calculating the amount of steam needed to open a check valve. Say I have a volume of 240cm3 with a pressure release valve of 38 bar and the temperature of steam is 247C and the heat loss is about 14 kilowatts per second.

thanks

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#1

Re: Steam is a small volume

01/25/2012 3:25 PM

You might start here.↓ BTW, we don't do homework here.

Download Free Steam Tables, Steam Tables 6.1 Download

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#2
In reply to #1

Re: Steam is a small volume

01/25/2012 3:32 PM

this is not homework

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#3
In reply to #2

Re: Steam is a small volume

01/25/2012 3:44 PM

Well, that's good. The way the question was phrased made me wonder.

Did you look at the steam tables?

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#4

Re: Steam is a small volume

01/25/2012 4:05 PM

heat loss is about 14 kilowatts per second.
Kilowatts per second is not a measure of heat loss.
Kilowatt-hours per second could be a measure of heat loss, but that would be an incredibly high rate for such a small volume.
The release valve will open with a small amount of steam. That small amount of steam will then pass through the valve to be replaced with another small amount of steam.
This might help.

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#8
In reply to #4

Re: Steam is a small volume

01/26/2012 5:05 AM

Sorry it is heat flux I am using Fouries law of conduction to calculate heat loss. And the valve will only open when the pressure in the vessel has reached the cracking pressure point.

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#11
In reply to #8

Re: Steam is a small volume

01/26/2012 5:14 AM

One minute it was a check valve, the next it is a relief valve.

The units of heat flux are energy per unit area per unit time.

What's going on?

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#5

Re: Steam Is A Small Volume

01/25/2012 11:56 PM

I'm confused by your use of the term "check valve". A check valve is a one way valve and any volume flowing in the correct direction will open it (ignoring the "cracking" pressure of the check). Please explain the application and whether you are looking for volume of flow or "cracking" pressure.

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#7
In reply to #5

Re: Steam Is A Small Volume

01/26/2012 4:24 AM

I am looking for "cracking" pressure for a valve.

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#12
In reply to #7

Re: Steam Is A Small Volume

01/26/2012 5:18 AM

If it's a relief valve, then the cracking pressure will be that indicated in the set-up record document for the valve. If using steam or any other gas, the item will be subject to an inspection by a representative from the firm supplying burst liability indemnity insurance for the pressure system, so the cracking pressure will be stated on the test record document that this organisation produces.

Why is the forum involved with this at all?

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#13
In reply to #12

Re: Steam Is A Small Volume

01/26/2012 7:58 AM

My question is how many kg's or steam will open the valve if the volume is 240cm3 and the relief valve is 38 bars the temperature of the steam entering the vessel is 250 degrees C. The vessel is a heat exchanger with an area of 0.24m2

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#14
In reply to #13

Re: Steam Is A Small Volume

01/26/2012 8:17 AM

There you go again. kg's is not a unit of pressure.

I'm afraid you are terribly confused.

Did you look at the tools already provided here, or do you just want the answer, without regard to how it was derived?

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#17
In reply to #14

Re: Steam Is A Small Volume

01/26/2012 10:54 AM

It depends on how one defines pressure and the method used to arrive to a particular unit of pressure. Any unit can represent pressure think about it. I have steam tables and programs for calculating them. The problem I have is in determining the amount of steam needed to open a relief valve in a vessel that is smaller than a cubic meter. I do not need the answer given to me, "although that would be great" but some help in calculating the result that I need, because I have been dealing with this problem for some time. Should I use the specific volume of the steam. I am at a loss and need help on this steam is not my thing but I still need the answer

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#19
In reply to #17

Re: Steam Is A Small Volume

01/26/2012 12:45 PM

Specific volume is the reciprocal of density. If the volume of the system is known, the mass of steam at any particular pressure and temperature drops out of some simple arithmetic.

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#16
In reply to #13

Re: Steam Is A Small Volume

01/26/2012 10:23 AM

Use the steam tables. That's what they are for.

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#15
In reply to #5

Re: Steam Is A Small Volume

01/26/2012 8:53 AM

Why did someone give me a GA for asking a question???

This is too confusing for my tired brain!!!

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#6

Re: Steam Is A Small Volume

01/26/2012 3:30 AM

The amount of steam needed is that required to achieve sufficient pressure difference in the system to open the flap inside the valve.

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#9
In reply to #6

Re: Steam Is A Small Volume

01/26/2012 5:07 AM

yes

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#10
In reply to #9

Re: Steam Is A Small Volume

01/26/2012 5:13 AM

That's it. Everything else responds to calculation.

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#18

Re: Steam Is A Small Volume

01/26/2012 10:58 AM

I would try to formulate the question as I think I understood your problem, of course I may be wrong :

If the valve opens (38 bar) and the steam flowing through is at 247 °C how big would be the flow if the heat ingress is 14 kW to maintain the valve open?

The answer is it depends on the valve characteristic since after opening the pressure drop increases proportional to the flow. In fact the opening is maintained by a balance between pressure forces (opening) and spring and flow forces (closing).

Or you can say that you only want to know the flow of steam at 247°C and 38 bar whose power is 14 kW. that you can do with recommended steam tables and compute the energy per kg and then the nr. of kg/s to have the power.

The problem with CR4 is that people are English speaking as mother and father language so that they have some troubles to put themselves in the shoes of those who do not use the language in a perfect mode.

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#20
In reply to #18

Re: Steam Is A Small Volume

01/26/2012 3:05 PM

l have calculated the mass flow. This is what i would like to know. I have a pressure vessel that has a volume of 240m3 and a valve that is 38 bars, how much steam will open the valve. l need to know the amount of steam. Thank you for trying to understand my question

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#22
In reply to #20

Re: Steam Is A Small Volume

01/26/2012 10:35 PM

OK, I admit I am semi literate. I seem to recall that steam relief valves will have an un-seating pressure or set point, and a re-seating pressure. The mass flow of steam will depend on the time to change from the unseating pressure to the re-seating pressure.

Also, the mass flow will depend on the Cv of the relief valve configuration.

I reserve the right to be completely wrong on this. What do the experts think.

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#21

Re: Steam Is A Small Volume

01/26/2012 7:53 PM

Volume and flow do not matter when talking about cracking pressure... only the pressure matters.

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#23

Re: Steam Is A Small Volume

01/28/2012 11:42 AM

As I understand your question you have some sort of boiler with a steam volume of about the size of a standard cup of coffee and heat input of 14kW, max working pressure is 38 bar.

Safety relief valves for steam are set for two values of pressure: the opening pressure and the lower blowdown pressure which allow the relief valve to reseat with a snap.

If you are trying to find out how much steam will be released when the relief valve opens the answer depends on the reason it opened in the first place, i.e. did the boiler stop valve suddenly trip, cutting off the only other way out. In that case the relief valve will open and stay open until the heat input is stopped and the pressure has been reduced to the blowdown pressure. Or was there just a minor load reduction or loss of heat input control requiring only a short pop to reduce the pressure to the blowdown value.

Based on the values you gave, in the case of boiler trip I would estimate a release of about 0.282 kg/s as long as the 14 kW are input.

By the way, what are you using this for, the steam quality must be pretty poor with such a small volume.

Hope this helps

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