Vacuum Tank Water Drain Design

[Okieng]

I am installing a mist elimination separator tank on the inlet piping of a vacuum exhauster. The separator will be operating at 13" Hg. The separator has three cleaning showers that will clean off mist elimination screens inside the tank. I need to install a drain line. My question is how to design a p-trap or low point drain to keep the vessel sealed?

 

[quark]

Are you sure of a p trap? You have to have about 5 mtrs of height below the separator(if I presume 13"Hg is gauge vacuum). If so, the liquid seal should be 4.5meters approximately with water. Mercury as a sealing liquid is ruled out in your case

I would suggest to put an actuated valve interlocked with pump(valve opens when pump is off).

Regards,

 

[Okieng]

Thanks Quark, The tank is to be mounted on a second floor mezzanine. There is a seal pit in the basement level, I was wondering if I could run the drain pipe down to the seal pit. The drain line is 3" dia, the shower water volume will be approximately 25 gpm.

I'm not sure taht I can make a 4.5 meter P-trap work.

Will the seal pit idea be sufficient?

Thanks for your repsonse.

 

[ApC2Kp]

Okieng,
The separator tank will need to be 5 m higher than the drain outlet, as described by quark, for a drain to atmospheric pressure during continuous operation of the vacuum exhauster. It seems the second floor mezzanine does not give you nearly enough height for water head to overcome the partial vacuum of separator tank.
If you could accept intermittent drain operation for low volumes of drain flow, then a "vacuum lock" chamber with three (actuated?) valves could allow draining from the separator tank with less height.
A "vacuum lock" chamber would function like the airlock doors between buildings with a controlled atmosphere, or the docking passage way between space vehicles connecting while in the vacuum of space orbit. A block valve, a drain valve, and a vent valve together on a length of piping would form the vacuum lock chamber. The drain and vent valves would be closed while the block valve remained open to fill the chamber with drainage from the mist elimination separator tank. When the chamber is to be drained, the block valve would first be closed, then the vent and the drain outlet valve would be opened. The block valve and the vent valve might be combined into a 3-way valve to prevent reverse flow if block valve was opened in error. The pipe could be run in horizontal, but it would drain better if oriented vertically. The available height will have to determine the design along with the required volume for drainage and drain cycle periods. The actuated valves could be sequenced with the cleaning showers.
If there is large volume of drainage to be sent to a waste treatment header (instead of process sewer / floor drain) then you might check into the condensate return units from Armstrong or Sarco that use steam pressure to push condensate into condensate collection header piping. Your application would need compressed air to push the fluid to waste treatment header. If the drainage volume is small, but it still needs to be 'pumped' to waste treatment, then consider connecting compressed air to the vent valve of the vacuum lock chamber, and connect vacuum chamber drain valve to the waste treatment header.

 

[rmw]

I believe that

http://www.graham-mfg.com

has some technical papers on its site regarding the design of drains from vessels operating under vacuum. Go root around on that site and see what you can find.

rmw

 

[quark]

If you have 5 meters between the bottom of the separator and top water level of the seal pit, this will work. Ejectors with barometric condensers use the same principle. You should always take care to dip the separator outlet pipe below the water level in the seal pit. This can be ensured by overflowing the drain above the dip pipe.

You can also give a thought to ApC2Kp's idea.

Regards,

 

[25362]

The friction drop for a 3" pipe for (gas-free) clean water at ambient temperature, say 25^oC, flowing at 25 gpm would be about 0.2 ft/100 ft, negligible for the case in hand and quark may be right.

However some issues would have to be clarified before taking a decision. For example:

What will the draining water temperature be and what would be the "dirty" water composition (dissolved or foamy gases, liquids or solids -sticky or not-) after cleaning the demisters ?

Kindly note that the water stream density, viscosity and vapor pressure are factors in fixing the minimum length of the "barometric" leg.

Depending on the answers to those queries you might be forced to use a device as ApC2Kp indicated or a pump.

 

[Okieng]

The contaminants are mostly paper fibers, and are washable. The issue we are experiencing is the fibers building up on the high speed blower impellers and creating an imbalance. We shut the equipment down and internally wash the unit and return vibration levels to normal.

The area on the mezzanine that we are locating the separator is located directly above a floor drain. The floor drain runs through the main floor and into a U-drain in the basement. Can I use the floor drain piping for the separator and rework the discharge into the U-drain to seal the piping?

I found a technical paper on the Graham website that details a separator drain design. Thanks. I will also investigate the possibility fo using the valve arrangement.

Thanks for everyone's suggestions so far, I appreciate the help.