Safety valve underpressure oil tank

[Yobbo]

L.S.,

I am confronted with a challenge to find some kind of safety valve, that keeps the underpressure in an oil tank of a gasturbine within limits. I will clarify the need with the following.
In order to return lubrication oil from the bearings to the oil tank an underpressure is being maintained to a value of 500 Pa under ambient air pressure. Fact is though that depending on the power of the machine there is a variation in the leakage air flow disturbing the value of the underpressure in the tank. If the underpressure becomes too low the oil tank might implode. That I want to prevent at all costs.
I either have to control the speed of the side channel fan to control the underpressure OR let the fan run at constant speed and prevent the underpressure from getting too high by applying some kind of safety valve, that opens when the underpressure exceeds a certain limit. The lower limit is about 1100 Pa below ambient pressure. I did contact a supplier who has control valves without external energy supply in his assortment, but he tells me that this pressure difference between ambient and internal pressure in the oil tank is rather low to be able to let such a valve work.

Does anyone of you have an idea or a hint, where I can look to find a usefull product, that can comply with my requirements?

Thanking you in advance for your effort I remain,

With best regards,

Karel Postulart

Karel Postulart, The Netherlands
Nuon Power Generation

 

[Yobbo]

Let me add that the underpressure may never come nearer to the ambient pressure than -300 Pa. This might lead to stagnation in the flow of oil back to the lubrication oil tank en eventually lead to fire.

Karel Postulart, The Netherlands
Nuon Power Generation

 

[Yobbo]

And a schematic representation of the situation

Karel Postulart, The Netherlands
Nuon Power Generation

 

[CRG]

You could do it either way control valve or VFD. I would look at a redundant system where you could measure the differential pressure and control the fans speed and then have an additional differential pressure transmitter operation a pneumatic control valve. Look at a Rosemount 3051 with a -3" H2O to 3" H2O range. This really should not be a problem provided you have instrument air for the pneumatics.

 

[CRG]

How about a weighted damper similar to what is used in the HVAC industry? This could be the simplest solution.

 

[gerhardl]

You can either do as proposed by some of the posts above, or you could do as you have proposed yourself as a possibillity: prevent a too high (or rather low) underpressure by mechanical weighted vacuum breakers.

Any mechanical device preventing underpressure will have a certain 'activating and repetitive' tolerance. You have to be very precise in defining the limits (upper/lower) where a 'vacuum breaker' could operate.

One of the complications is that you have to maintain the underpressure at not nearer atmospheric than minus 300pa, and have to avoid coming down to minus 1100pa.

This will leave you only an 800pa band to work with, including tolerance limits.

Suggestion: Check if lower limit could actually be lower. The figure seems high as compared to what a normal vessel could tolerate, and you might have better possibillities with a broader working range, if vessel and process could tolerate this.

Further suggestion: Check following link:

http://www.norgren.com/document_resources/EN/en_press_swit.pdf

A simple pressure switch for vacuum (18D - type for this example) will have a 'typical' non adjustable pressure difference of about 0,004 bar (400 pa) and could teoretically be used to operate a pneumatic ball or seat valve on/off letting air in between your limits, acting as a vacuum breaker. If this works (cheap to test with actual equipment), this will let you run the fan continously without realatively complicated/costly equipment.

To adjust you should both calculate necessary Cv for the vacuum breaking valve and have a manual (lockable?) control valve in series for final adjustment at site. You also have to be careful where you place the pressure switch (a place which is undisturbed by turbulence from fan and air inlet). All this to obtain as smooth operation and as few adjustments as possible.