Steam quality from a boiler 1

[Canoman]

Does anyone know of a good method to calculate the quality of steam leaving the disengagement space of a waste heat boiler? The boiler recovers heat from reactor effluent and produces steam to cool the process gas.

I am looking for a method on how to figure out how much water is entrained in the steam.

Thanks

 

[butelja]

Throttling calorimeter is the standard method.

 

[Canoman]

"Throttling calorimeter"

This seems to sound to me like a way of measuring the quality. I need a way to calculate this. How about an empirical method?

Anyone know of any books/articles about something similar to this?

 

[butelja]

In a throttling calorimeter, the steam is throttled to atmospheric pressure. The throttling process is isenthalphic, meaning that the enthalpy of the steam is the same at the high and low pressure. Based upon the temperature after the throttling valve, the steam quality can be calculated using a steam table.

If the upstream condition is denoted as "1" and the downstream condition is denoted as "2", then

X1 = (h2-hf1) / hfg1, where

X1 = quality of steam at high pressure
hf1 = enthalpy of saturated liquid at high pressure
hfg1 = enthalpy of vaporization at high pressure
h2 = enthalpy of (superheated) throttled steam at low pressure

h2 is looked up in a steam table based upon the temperature reading from the throttling calorimeter.

I hope this helps.

 

[Canoman]

This makes sense to me, but I have no access to a throttling calorimeter.

 

[butelja]

All you need is a small tap (1/2" or 3/4&quot with a globe or needle valve. Add a short section of piping connected downstream of the valve with an accurate thermometer in it, discharging to atmosphere in a safe place. Insulate the whole affair. Now you have a throttling calorimeter.

 

[Canoman]

Hmm, interesting. My problem is that I'm a process engineer in a consulting firm - not out at the plant. I need a method to calculate the quality for things like a P&ID review. If the client is looking over P&ID's and asks what quality steam our boiler will produce, we need to be able to tell them so they can decide whether or not to spend the money to install a mesh-type mist eliminator in the boiler.

Sorry if I was a little unclear in making it sound like I was at the plant. I found out this morning that it's not so much to help the operation of this boiler, but to be able to provide a value of the steam that will be produced before the boiler is even in operation.

 

[butelja]

Hmm. This sounds more like a question for the boiler manufacturer. The answer will probably vary with the loading. I was of the impression that most boilers have either mesh pad or cyclonic type mist eliminators in the steam drums.

 

[Canoman]

I agree... My manager has contacted some boiler manufacturers and mesh pad manufacturers. They told him that they have a way to calculate this, but they will not disclose it.

*grumble grumble*

 

[Ahmedv]

Hi

You need to use the Souders-Brown equation to calculate entrainment in your system. Go to Amistco.com, they explain it nicely.

Regards Ahmed Vawda
Process Engineer
Al Khaleej Sugar - Dubai
avawda@aksugar.co.ae

 

[Canoman]

The use of Souders and Brown in this situation looks interesting. I have never used that for anything but distillation columns. Anyhow, that correlation will give me the velocity at which entrainment of a certain droplet size occurs, but no real way to estimate the quality of the steam. If I could somehow get a rate of formation of these droplets, then perhaps I could combine that calculation with Souders and Brown and come up with a rough estimate of how much entrained liquid is leaving at the given velocity.

I have been looking, but have not been able to find anything that will allow me to estimate this rate. There are so many variables involved in that, that it may be impossible to calculate. Splashing within the boiler, surface tension, gas velocity, distance from where the droplet forms to the outlet, etc. etc. etc. all affect the quality. I am not aware of a method to calculate how much all of these things contribute.

 

[ArtR]

Very interesting discussion.

What is the steam used for? If it is only for process use, the normal boiler discharge of 99.5% quality should be OK if your header trapping stations are properly designed (big connections to prevent the condensate from "flying over" the drip pot).

But, if the steam goes to a turbine, it must be very dry.

Doesn't the boiler have an internal separator?

 

[Canoman]

The boiler has no separator. It was designed for enough volume to allow for gravitational separation. I am not sure of the use of the steam... They have poor enough quality that they are getting knocking in their pipes, so says my manager.

We are pretty sure that boiler manufacturers make this calculation, but so far we have not found anyone who is willing to give us the method of doing this calculation.

If I could calculate the amount of entrained liquid, then my problem would be solved (I think).

 

By far, the easiest method of determining steam quality is the use of a calorimeter. Butelja is correct. All you need is a decent place to tap in. It takes five minutes. I have one in the trunk of my car (I use it for steam ejector inspections and troubleshooting gigs).

I doubt you'd get an accurate number from just manufacturer's data. Too many desk-bound assumptions. You simply have to go measure it. It's just one of those things. Plus, the manufacturer won't know anything about the field conditions (effectiveness of insulation, velocity, water chemistry, etc).

Whenever I have to convince a client he has a quality problem, I measure it at the boiler, half-way down the main, and at the user. Works every time.

Yours truly,
Steam Ejector P.E.
Serving the Western Hemisphere

 

[Canoman]

Steam Ejector P.E.,

I agree that there is no more accurate way of doing this than measuring it, but I need a way to calculate what this should be, even before the boiler is built. If a client comes to us wanting to know if they should spend the extra money to install a mesh mist-pad or other type of droplet eliminator, then we should be able to tell them whether it is worth their money or if it is not.

Canoman

 

[butelja]

Canoman,

You mention that the situation is bad enough to cause knocking in the piping. This problem may or may not be originating in the boiler. If the steam line is not adequately insulated and/or the piping does not have adequate drip legs and traps, then condensation accumulating in the piping downstream of the boiler can cause knocking. This can occur even if the quality of the steam leaving the boiler is 100%.

 

[Canoman]

Yes, I think though, that the existing traps are sized correctly for our original design. The problem is that they are operating with a water level above what we designed for, which increases the velocity of the exiting stream, therefore increasing the amount of entrainment. If they lower the liquid level to what we specified, it should work fine.

We've gotten a little off topic here. My main concern is figuring out how to calculate a steam quality for a designed, but not existing boiler. How can I do a somewhat accurate calculation to figure out the quality of steam that a newly designed boiler *should* produce? We are almost certain that boiler manufacturers have a way of doing this since steam quality needed for some pieces of equipment is so crucial. We just can't figure out how they do it, and they are not willing to disclose that information.

 

[butelja]

You may want to consider installing an inline centrifugal separator on the boiler discharge. Wright Austin makes a good unit that they claim will remove 99% of entrained liquid. I believe that they are owned by Hayward now.