Vacuum Drop - Code Requirements

The maximum vacuum is that which is related to the temperature of the condensate. So if you warm up the condensate then the vacuum will be lower.

Have you a set of steam tables to hand?

Your reply is understood.

However, I am talking about RATE of Vacuum drop, i.e. mbar/min or inHg/ min.

This indicates as how leak-proof the plant vacuum system is and on loss of Vacuum pump, how long the turbine protection will take to become operative !

This RATE of vacuum drop is defined in HEI standard and my querry is related to the acceptable norm as per standard. From a normal vacuum of 29" Hg, it may take 20-25min when the vacuum level may reach the turbine protection set value.

Hi,

Your steam turbine will have been designed to operate into a specific vacuum. Any departure from this will reduce its efficiency according to a relationship between the starting steam pressure and the vacuum; basically you are looking at an operating range between 26 and 29 ''Hg. The poorer vacuum should be treated as a cut-off point where as anything above 29''Hg is a bonus(remember the effects of atmospheric pressure variation when watching a vac gauge reading.

You should therefore check with the turbine manufacturer to see what the design vacuum was and then set your evacuation system to cycle around that figure.

As the vacuum becomes weaker so the specific consumption of the turbine will increase so there is a penalty for operating with a low vacuum.

Hope this helps.

I appreciate your response. However, my querry was slightly different in nature.

Let me repeat the problem. We would be stopping the Vacuum pump at full load which would result in gradual drop in vacuum. Under this operating regime we intend to check the rate of drop in vacuum. This is not a normal operating condition.

QUERRY: What is the accepted rate of vacuum drop as per HEI code?

Hi SKP,

I am not au fait with the HEI code and their website appears difficult to access. So can you give me the gist of the code in respect of this, please.

I do question the reason for what you are trying to achieve; is it because you suspect that your evacuator is having to do too much work? If so then you are trying to decide what action to take to reduce the quantity of air which is entering the steam system. Do you operate the gen plant on a 24/7 basis?

If you are experiencing a greater air problem now than in the past is it following an overhaul of the turbine? I'm thinking of mal-fitting shaft seals.Have you checked how the de-aerator(s) are working?

Look forward to hearing from you.