Steam drum area of disengagement 5

[curve3104]

A certain area of water surface is required in a boiler from which to release the steam at an acceptable velocity with minimal carryover. I am looking for design standards or calculation methods for determining steam drum diameters and lenghts to ensure that the area of the normal water level would be acceptable for a given steam load.
Anyone know where I could find these answers or does anyone have experience with this topic?
Thanks!

 

[athomas236]

As far as I am aware there are no published hold up times. Sometimes a consultant will include in the boiler spec a requirement for a hold up time of one minute but these days with turnkey contracts for the whole power plant this practice is becoming rare.

I have never seen a hold up time of ten minutes, not on water tube boilers any way.

With the advent of HRSG's I have seen requirements that the storage volume between NWL and HWL trip should not be less than the volume of the evaporator.This is to prevent HWL trips when the GT starts.

athomas236

 

[athomas236]

More details from a recent turnkey project in Europe as follows.

Boiler MCR flow = 116.7kg/s
Drum pressure at MCR = 147.2 bar
Specific volume of water at 147.2 bar = 0.01065 m^3/kg

Drum sizes = 15m long x 1.528m ID
NWL is 100 mm below cenre line
LWL trip is 300 mm below centre line

Volume between NWL anf LWLTrip = 4.45m^3

Hold up time = 4.58/(.01065*116.7)=3.7 sec

athomas236

 

[25362]

To athomas236

1. From the above data the NWL in this project, would represent about 42% of the total drum volume (11.5 out of 27.5 m³).

2. The water specific volume at saturation would be 0.00164, not as given. Thus the estimated volume between NWL and LWL of about 4.5 m³ would correspond to about: (4.5)/(0.00164x116.7) = 23.5 seconds.

3. The total water holdup at NWL of 11.5 m³ would correspond to about 60 seconds (1 min).

4. Question: is the plant provided with alternative steam/power supplies ? Thanks.

 

[athomas236]

25362

Thanks for checking the calcs, used sat vapour in error. Have a star.

Plant has no alternative steam or power supplies, it is a 2 unit cfb plant.

Regards and thanks again

athomas236

 

[25362]

Beside the considerations on possible BFW pump failure, the level of water in a steam drum is more difficult to control at low pressures because of the larger difference in densities between water and steam.

For example, at 10 bar the ratio of densities is around 175:1, while at the pressures of the example brought by athomas236 is only about 7:1.

Thus, "shrink-and-swell" effects due to pressure (load) changes are quite large at lower pressures. In these cases, level controllers need a wide (~100%) proportional band and several minutes integral time. And probably more "residence" time.

This is to show that many variables play a role in the design of steam drums.

 

[athomas236]

When we talk about drum water levels we have to be careful what we mean. There is the "actual" level in the drum which in many operating conditions is turbulent steam/water surface and there is the level in the gauge glass/water column which are either what we see or what the instruments indicate. This second level is what initiate alarms and trips. This level is generally less prone to sharp changes because it is the equivalent "solid" height of the steam and water mixture in the drum.

Just a thought

athomas236

 

[25362]

Athomas236 is right. Level glasses tend to show a "negative" error (that needs correction) because of "cooling". Anyway, "shrink-and-swell" effects would be transmitted to the gages.

It would be interesting to see whether natural and forced circulation water-tube boilers use the same criteria for sizing steam drums.

 

[athomas236]

The answer is that the drums for natural and assisted circulation boilers are designed to the same manufacturer's standards in terms of cyclone loadings. This may result in more cyclones for the natural circulation boiler than the equivalent assisted type.

The reason is that the number of cyclones is commonly based on a factor x steam flow or another factor x the recirculated water flow, whichever gives the highest number rules.

I agree with 25362 that swell effects will be transmitted to the gauge glasses. What I was trying to say was that a 100mm swell in the steam and water mixture will not translate to a 100mm swell in the gauge glass because of the different densities.

athomas236

 

[davefitz]

For high pressure natural circulation boilers,one design standard is to provide sufficient cyclone /centrifugal separators such that the pressure drop through them is limited to less than 5 ft liquid ( at operating pressure).

For determining the number of chevron driers, the pressure drop or momentum thru the driers must be limited to prevent carryover of water droplets to the steam outlet nozzle; such carrryover would cause exceedance of permited steam purity since the water impurities would contribute to solids deposition at the steam turbine.

Athe minimum number of centrifugal spearators and driers is determined, then you must figure out the smalles diameter drum which can fit all this equipment and still have enough room to crwl thru.