Superheated steam in Partial Vacuum 1

[code1]

I am pretty rusty in Thermodynamics, and I have spent time reading my old texts, but no luck to find a classical text-book solution:

What will the resulting fluid properties be if, 1200kg/h SH steam at 0.8 bar(a) and 340 deg. C is allowed to flow into a exhuast steam duct kept at 0.8 bar(a), 93.5 deg. C (saturated temperature) by means of exhausting to the vacuum pump unit and the condenser.

Will the SH steam 'flash', cool down to 93.5 deg. C? Will the duct heatup to 340 deg. C.

I am unable to formulate the thermodynamic equations. Any help/ hints greatly appreciated.

This condition is encountered during the turbine startup heating.

 

[steamdog]

I am rusty, too, on my Mollier curve for this type of pressure reduction. However, your pressure drop is very small, and you will not get much change in temperature of the steam. You can only 'flash' when there is water present, so the superheated steam will stay pretty much at the same temperature.

 

[insult2injury]

Your condenser circ water must remove enough heat to condense the incoming steam. If the condenser load increases such that there is not enough surface area or circ water flow to remove all the heat, the pressure in the condenser will increase. This should not be the case if all you are considering is small flow from warm up of piping/components.

The exhaust steam appears to be rather hot for warmup. The steam will cool/condense as the components are warmed.

I2I

 

[code1]

Typically during running, the turbine (condensing) is at 0.8 bar(a) exhaust. Flow rate is ~180ton/hr. So the condenser load is more than sufficiently provided.

However, surprisingly, and unexpectedly, the turbine drains will be SH at 340 deg. C at the end of the initial heating (Steam conditions during running is 410 deg. C, 43 bar). Causing me some design headaches, because the initial mechnical design temperature is 150 deg. C only for the drains system.

This is why I need to refine the problem thermodynamically to see if I can obtain any lower process temperature by means of thermodynamics calculation instead of applying a 360 deg. C Mechnical design temperature. Hence this post to the forum members.

 

[insult2injury]

Are there automatic drain valves, steam traps, or loop seals provided?

I2I

 

[stgrme]

Drains from the turbine are usually designed for high temperature. Typically, the turbine manufacturer will not close drain valves until it is certain that no condensing will occur within the the turbine casing. In other words, you will have steam flowing through the drain valves before they are closed and you must design the drain system to handle this steam. Because steam turbines are generally inefficient at low throttle flows and low loads, temperatures at the drains can approach the inlet steam temperature.

Best of luck!

 

[chicopee]

Do an enthalpy balance m1h1+m2h2=(m1+m1)h3 to calculate h3 and from steam table for the p3=.8bar(a) and h3 find out T3,v3,s3

 

[code1]

I have considered to do the mass and energy balance, but as the superheated steam (m1, h1) mixes with the steam in the duct (m2,h2), what volume/ mass of m2 should be considered? 1, 5, 10 m of steam in the duct?

Any consideration on any irrecoverable work done by the SH steam

 

[chicopee]

By the way ckeking back on my first reply I got a misprint being (m1+m1)h3 which s/b (m1+m2)h3; m1,m2 and m3 are mass rates (lbm/hr)and m3 = m1+m2.

 

[chicopee]

It will depend how much steam is flowing in the exhaust steam duct. Obviously your answer will lie between 93.5 and 340 d.C..You need to know one more steam property to answer your question and probably, the easiest would be the mass rate of the liquid at 0.8 b,93.5 d.C. which in my above reply would be m2.

 

[rmw]

As I understand the original question, your duct is flowing to the main condenser which has the normal air removal (vacuum) equipment. If so, during the start up process the condenser has lots more surface area than is required for the reduced steam loads at start up and even the drains. For some large turbines there are VERY many drains and some of them come from areas of the turbine where the steam is very very hot.

The superheated steam will heat up the saturated steam to some value above saturation, but the massive area of the condenser can handle that at that point.

It is when the turbine is at full load and there are leaky valves on those drain circuits that bring excessive drains and drips into the turbine that hurt.

For the record, the vacuum equipment doesn't create the vacuum, it just removes the non condensables, mainly air. The condensation of the steam and the change of state of the steam from a vapor to water is what creates the vacuum. As long as there is enough condenser surface area to condense the steam, the vacuum will be created and maintained. The superheated steam, that heated above saturation by the drain inlet flow will have to be cooled to the saturation point for the condenser pressure by sensible heat transfer at which time it will then condense and contribute to the vacuum creation.

So as long as this happens during the warm up/start up part of the cycle, when the steam flows are light, you should be OK. Remember too that depending on lots of factors in your turbine it is possible that the steam from the turbine is entering the condenser with moisture in it. Some of the superheated steam that you are admitting via the drains flow will just bake some of that moisture off. If there is enough moisture present to balance the added heat of the SH drain steam, you won't raise the condenser pressure.

I hope this helps answer what you are looking for.

I have seen leaky drains that were so hot that they warped the walls of the condenser adjacent to the point at which the drain connection was connected. Often drains of this nature are located under the water level of the hotwell and others are located under the condenser bundles so that the SH steam can be quenched by the water droplets falling from the bundle. That helps with the process of raising the condensate temperature to prevent condensate sub cooling.

rmw

 

[code1]

Dear all,

I apologise for my late reply as I had been travelling.

In any case, for closure to this thread, the mech. design temperature of the system is only 150 deg. C and as such, in order to maintain the warranty on the design of the system and for safety, I have decided to go for desuperheating to less than 100 deg. C for the case mentioned above.

The last minute addition for the above is costing top dollars, but necessary, nevertheless.

Thanks, again, all for the many valuable inputs and comments!