Velocity at exit of boiler (cylinder) 1

[deangardner]

I have an interesting problem that a few of us in the office have been struggling with.

You have a simple water storage cylinder that is heated to produce steam. It flows through 4" pipework to a pump some 200 meters away and is then exhausted into atmosphere. Trying to work out what the steady state pressure would be x point along the pipe. This seems to rely on knowing the velocity at the 4" exhaust from the boiler. You can calculate this from knowing P1 at the Pump and P2 at the boilder, but the boiler also acts as a pump and will add some additional velocity to the system. The pump is 40m3 at atmospheric conditions and pumps down to 0.001mbar where the velocity will be 2m3/hour.

Essentially we have two pumps in series with no pump curve for the steam generator...

Any help would be greatly appreciated.

 

[Dougt115]

IF you are heating a closed tank to create steam then you will not know the pressure without knowing the temperature. If it is an open tank then the pressure is atmospheric.

What flows through the 4 inch pipe the water or the steam? 200 meters for the steam will likely result in condensation unless the pipe is insulated or heated.

What is 40m3? 40 cubic meters? Is that the tank size or the pump volume per hour? It pumps down to .001mbars with a rate of 2 cubic meters/hour.

So you are producing steam and pumping it down to .000001 atm? Can you do that? Room temperature water will boil at that pressure.

 

[deangardner]

Okay the temp the boiler is being heated to is 200 degrees c. It is trace heated at 150 degrees. the pump at 200 meters away has a pump rate of 40 meters cubed per hour at atmospheric conditions and 2 meters cubed per hour at it's ultimate pumping pressure which is 0.001 mbar. The pump will have a continuous pressure from the boiler at 200 degrees C and will not be able to reach 0.001 mbar, and there lies my problem. What will be the steady state pressure though the 4" pipe?

 

[btrueblood]

It will depend on how many watts you put into the heater for the boiler, those watts will be consumed by the phase transition from liquid to vapor (steam), some 2260 kJ/kg or thereabouts (enthalpy of vaporization of water).

 

[chicopee]

Is the steam generating unit direct or indirect fired? Of coarse the temperature, pressure and velocity will drop due to friction loss and heat transfer thru the pipe wall. The only constant is the mass rate.You'll have to do a spreadsheet and break down the pipe into equal pipe segements and it appears from you OP that you'll have to work backwards starting from the pump intake and ending at the steam generating unit outlet. If you know the operating pressure and the steam output of the steam generating unit then you can calculate in the reverse direction.

 

[IRstuff]

Please refrain from double posting

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[TD2K]

A sketch would help because I'm confused exactly where these various flow rates apply to in this system.

If the boiler is producing steam at 200C then the pressure can be read off the steam tables. If the boiler was producing at a lower pressure then the temperature would be lower. From your description, it seems fair to assume the steam is saturated at the boiler.

Then you need to know the mass flow rate of the steam to the pump (driver?). Then it's a simple matter of calculating the pressure drop through the 4" piping given the ID, roughness, length, number of fittings, etc.

 

[LittleInch]

So far what I can gather is this.

You have a boiler generating steam at 200C and at the saturated pressure (14.5barg)
You have a 4" pipe 200m long, insulated
Your flow into the pump is 40 sm3/hr (standard at atmospheric)

Now this is where it gets confusing - what pressure is the pump inlet? - note that if the inlet is at 14.5barg and the flow a relatively small volumetric flowrate, the inlet pressure will be pretty close to 14.5barg.

This "pump" then somehow converts 40 sm3/hr to 2mm3/hr??

what has 0.001 mbar got to do with this?? Is it mbara or mbarg? Either way a pump increases pressure / head, it doesn't reduce it.

Your units, numbers and change of volume to velocity to pressure makes no sense at all and therefore I'm not surprised you can't calculate anything because what you're apparently trying to do is physically impossible.

Come back with some proper data and you might get somewhere, but currently this is not a feasible system to model with the data as provided.

My motto: Learn something new every day

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

 

[TheTick]

Can you measure the mass flow rate at any point? If you're at steady state, m'(in) = m'(out). Then you can get velocity by measuring the steam temp and pressure at the outlet and dividing by outlet area.

 

[rb1957]

"but the boiler also acts as a pump and will add some additional velocity to the system" ... really? not my area but i thought a boiler would increase pressure, the velocity out of the boiler depends on the pressure gradient, yes? no?

i think you need to know the pressure in the boiler.

i think you need to know how much work the pump is doing. is it adding energy into the steam or removing it?

is it reasonable to assume the exit conditions ? standard atmosphere with zero velocity ? it sounds like you need the exit velocity as well, but we might be able to deduce it with the other data.

another day in paradise, or is paradise one day closer ?

 

[deangardner]

The water pressure is roughly 20 bar at 200 degrees c. The pump on the end is a vac pump that pumps down to 0.001 mbar, and at this pressure the flowrate through the pump is 2 meters cubed per hour. If the boiler was isolated then the pump would be able to pull a good vac, albeit with some loss from the 200 meter offset. However, as soon as you open up the isolation valve there is a supply of 20 bar. If there was a pressure sensor at the vac pump inlet it would not be reading the ultimate vacuum pressure because of the boiler on the other end. There some "features" in the process line, it's not just truying to pull a vac on a boiler. There are other light gases formed in the process that are to be pulled out for safety. So if you isolate the boiler we can determine the pressure drop at the boiler and associated mass flow. However what happens when you open the isolation valve to the boiler with some amount of water in there that is boiling off?

 

[rb1957]

i`m confused ... is the pump input the boiler exhaust, and the pump exhausts to the outside ? if there are "other things" attached to the line (between the boiler and the pump) won`t they affect the outcome ? is the pump trying to reduce the intake pressure to 0.001mb ? if so, how can there be a flow through the pump (at this pressure) ? btw, at 2m3/hr through your 4" pipe the flow rate is 0.07m/sec = 0.2ft/sec; which isn`t much.

another day in paradise, or is paradise one day closer ?

 

[deangardner]

the flow is very slow at low pressure, correct. Don't worry about the bits in the way - yes they will have an affect but I'm not too concerned yet - until this works. The boiler is on one end - 200m away is a vac pump that pulls vapor through and is exhausted to the outside world.

 

[LittleInch]

Ok,

Now we have a completely different set of circumstances, but lets just go with it for now.

At 20 bar on the boiler you won't be making any steam as this is above the saturated vapour pressure, but you will have super heated water. You have a pipe which is now isolated form the boiler and a vacuum pump connected which has pulled the pressure down to some unknown pressure, but appears to be less than 1 bara. This vac pump pumps vapour out at what I can only assume is 2 scm/hr, though vac pumps don't have a fixed rate..

Now your question is "what happens when you open the isolation valve to the boiler with some amount of water in there that is boiling off". In short an extreme event is what happens as the pressure falls in the boiler to fill your pipe at near sonic velocity, the superheated water suddenly starts producing steam in a rather violent manner and the water in the pipe is then moved at a very large velocity towards your vac pump which would probably explode under the impact....

This assumes of course that when you open the isolation valve you do so with no throttling or control.

This doesn't sound like an operating scenario that you really want to consider, but then maybe I've misunderstood again what it is you're doing or trying to find....

My motto: Learn something new every day

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

 

[rb1957]

i think the saving grace in this is that as the valve is opened, the flow is certainly choked.

but how will the pump respond if it's trucking away, sucking vaccuum, and then all of a sudden it sees a significant pressure ?

another day in paradise, or is paradise one day closer ?

 

[LittleInch]

not only significant pressure, but the condensed steam which is sitting in the bottom of the pipe arriving at near sonic velocity....

My motto: Learn something new every day

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

 

[Dougt115]

The boiler pressure will push the steam down the pipe. Based on the pressure (temperature) and the pipe characteristics you should be able to determine what the flow rate would be.

The pump at the end of the pipe will pull the steam. Using the pipe configuration and the properties of a gas you should be able to determine what the flow rate would be.

Due to your pump being variable and the flow from the tank being variable, the two alter the performance of each other. You will not get an exact answer until the system reaches equilibrium and you can take measurements at the tank and pump.

If you really want a solution post this to the CFD, Computational Fluid Dynamics, group. CFD may be able to answer your question.

 

[rb1957]

i can`t see a steady state, with the pump pumping and the boiler boiling, and heat going all over the place (is the pipe lagged?) ...

it might be a little bit simpler if the boiler is off, and the pumnp (having created the vaccuum in the pipe, then at least there`s a steady state over the boiler (some volume at 20bar) and the opened pipe (some volume at 0.001mbar), then turn the pump on.

another day in paradise, or is paradise one day closer ?