Restriction Orifices

[MF34]

Good day.

A boiler feed water pump, has a minimum flow line back to a deaerator storage tank. In this min flow line there is a flow control valve and a orifice plate just before the Deaerator Storage tank. The pressure of the water upstream of the orifice is 41 Bar and the temperature is 210 degrees C. The problem is that downstream of the RO the line deteriorates.

I want to know how I can drop this pressure from 41 Bar to 5 bar, without flashing and with the temp of the water staying below the Saturation temperature of water at 5 bar.

I am a bit confused since the pressure drop that occurs over a RO happens adiabatic, and the temperature only changes over a RO if flashing occurs.

How do I then get the temperature of the water down so that it stays below the saturation temperature.

 

[hacksaw]

the usual purpose of the RO is to put back pressure on the valve to minimize cavitation (and valve damage), but the usual result is just as you have discovered.

you may need to consider a valve designed for high pressure letdown, they are not cheap and usually require major piping changes.

One alternative, is to consider using a series of a multiple port restriction plates to reduce the pressure in stages in a more or less controlled fashion. You feed water pump supplier is usally a good source of options on that.

Per your initial query, reducing the water temperature is only possble if you remove the heat, that generally is not an option.

 

[ione]

Saturation temperature for water at 5 barg is approx 158.9 °C. Your temperature upstream the RO is already well above that value, so you’re dealing with superheated water. Transformation is isenthalpic (not strictly but just taking into account initial and final condition), so no way to get what you’re looking for with an orifice nor with a control valve.

Suggest to have a look to the thread below

http://www.eng-tips.com/viewthread.cfm?qid=269963&page=25

 

[DRWeig]

Being just a little (but not totally) tongue-in-cheek, since I don't know the flow rate and the needs of the surrounding process, I just felt like throwing in a recovery system I have observed. You have an 8:1 pressure ratio -- use of a turbine to reduce the pressre while extracting a bunch of energy in the form of work might provide an additional benefit (the work). Sized right, you'll reduce the enthalpy (and thus temperature) enough to avoid flashing.

Note that I'm an electrical guy, I only did the controls for the example cited. I betya there are folks in this forum who could do the thermo in their heads though...

Good on ya,

Goober Dave

 

[MF34]

Thanks for the good replies.

With multiple orifice plates you will bring the pressure down in a controlled fashion, but you will still have flashing since you cannot bring the temperature of the superheated water down with a RO. The flowrate of the superheated water is 90kg/s, and the request is to bring the pressure down to 5 bar and that the temperature of the fluid stays below saturation temperature at all times. This however seems not to be possible, since you cannot remove heat with RO's. There must then be another solution to bring the pressure down, while also reducing the temperature, to prevent flashing. FOr example the turbine solution might not be that far fetched.

 

[ione]

Ok, so you’re aware water cannot exist in liquid state at 5 barg pressure and 210 °C temperature.
The solution is to use a heat exchanger to bring superheated water temperature below saturation temperature at 41 barg and then pressure reducer (orifice or valve) to reach 5 barg.

 

[katmar]

90 kg/s is an enormous flow for a minumum flow line. That flow rate with flashing could do quite a bit of damage I guess. If you can put it through a turbine you could recover about 200 to 250 kW. It may be more trouble than this little bit of power is worth.

One way of reducing the temperature and pressure at the same time would be to replace the orifice with an eductor. Use it to draw cold water in from your soft water make up tank. If the soft water is at 30°C you would have to draw in about 0.4 kg for every kg of 210°C water. A company like Graham manufacturing would be able to tell you if this is feasible.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[uu78hswang]

A group of orifice plates lined up in series with any combination can be used when the required pressure drop across one or more orifice plates would cause cavitation.

Put more orifice plates reduce the pressure drop
across each plate and it may help to prevent cavitation.

 

[uu78hswang]

Put a group of orifice plates with any combination are used when the required pressure drop across one or more orifice
plates would cause cavitation to occur.

Put additional orifice plates reduce the pressure drop
across each plate to prevent cavitation.

 

[sshep]

If you are going from 41barg to 5barg, there will always be flashing unless you cool the stream below 159C (saturation temp at 5barg) before dropping the pressure. As the water flashes it cools the liquid left behind until the saturation temp at 5barg (159C) is reached. No configuration of orifice plates can change this.

I calculate 11% vapor fraction going into the deaerator storage tank. The easiest thing to do is resize your line to handle the velocity of that sort of flashing. Drop the pressure right before the tank to minimize the length of the line. Some care should be taken in design and location of the control valves and orifice(s) in order to avoid localized impingment of a jet onto the pipe wall (i.e. give yourself some straight length on the downstream), but there is nothing that can be done to actually stop the flashing except by cooling to 159C prior to taking the pressure drop.

best wishes,
sshep