Pressure Gauge with Diaphragm Seal & Snubber

The pressure gauge is based on the deformation of a compliant element under pressure. If you introduce a membrane its stiffness will compromise the results. The problem is how to transmit with the lowest possible loss pressure to the sensing element. It can be done if you have a VERY deformable membrane and a stiff fluid between membrane and gauge. This solution is used by the Keller company in its gauges. As for the snubber since the transmission of pressure MUST be without losses it you have it the gauge life expectancy will be OK if not it will drastically decrease.

1. The diaphragm seal will not provide much dampening, so you may still need a snubber. It could be installed on the seal fluid to avoid corrosion/plugging.

2. Is there already a tap into the lined pipe? If not, how do you intend to do this?

3. Are you sure 316SS will resist your fluid?

4. Does the pump installation include pulsation dampeners?

We had similar problem of fluctuation/pulsation in fluid damaging diaphragm of Pressure switch.

This was the case of a fuel line of diesel engine. It is a 3600HP loco engine being used by Diesel Generating sets by Indian Railway in many of their factories in India.

Permanent solution was to increase diameter of fuel line and header so that it can hold enough diesel and work like a sink. (Probably, the fuel header and line size is same when same engine used to produce 2200HP against latest version which produces 3600HP.)

Diaphragm of Fuel pressure switch of one of the two DG Sets punctured. We had installed 1/4" ball valve for isolation in all instrumentation lines at root.

Pulsation snubbers are available in the market - but as a quick solution we slowly closed the ball valve to throttle it till pulsations in pressure gauge installed in same line vanished. This worked as an excellent devise as pressure pulse snubber. (For safety of DG sets, knobs of all root valves were removed and kept in key board with other keys of Control and Instrumentation system of DG Sets).

It is now more than a year and we have commissioned 3 installation above way -without another failure of pressure switch.

We are not instrumentation firm (basically small Electrical Power firm), first time participating on Instrumentation forum - members in Instrumentation field are most welcomed to accept or ignore above comments.

By closing the valve you created a 1st degree low-pass filter build up with the valve hydraulic resistance and the fluid volume between valve and gauge. This is equivalent to an R-C electronic combination. It is however sensitive to dirt and under circumstances can lead to a wrong reading.

What was NOT specified is if the pressure gauges are mechanic or electric. The oil filling which is a damper is ONLY for mechanical gauges valid.

As for the membranes their compliance plays an important role in the readings accuracy which ever the gauge type is.

Commercial pressure snubbers for instrumentation are typically a restricted flow path created by a needle valve, as shown in the illustration.

A snubber mounts on the gauge side of a diaphragm seal, between the seal and the gauge or pressure instrument, protected from the process by the seal's diaphragm.

lighting.arrestor-

First question: is the process fluid compatible with 316 SS? If yes, why use the isolation diaphragm seal. If no, you will have to find a PTFE coated diaphragm and the body for it with a PTFE coating or a machined body of PTFE since this item would probably be extremely hard to find if even sold already made.

The snubber: It too would have to be made of PTFE unless someone somewhere makes such a rare item in either Teflon coated or machined from PTFE. With a reciprocating pump this would be mandatory to be able to get any type of realistic reading off the gauge.

Another item that would do much more to reduce the pulsations to the gauge would be to add a pulsation diaphragm snubber to the output line from the reciprocating pump. A pulsation diaphragm snubber is identical to a hydronic heating system (circulated water to baseboard heaters or water radiators and then back to the boiler) expansion tank. It is also identical in function to the bladder type storage tank used in well water systems for residential use. Simply a tank with a sealed bladder of air across the middle or a sealed bag from the top end ( plain end) of the tank. As the line pressure increases, the bladder reduces in size to absorb the pulse of increasing pressure from the piston. As the piston retracts a pulse of declining pressure reduces the pressure on the bladder and it expands in size.. These units are regularly used on pneumatic powered single and double diaphragm pumps. Thousands and more are in use at this time. Only foreseeable problem might be finding one of a material compatible with the process fluid.

Good Luck, Old Salt

I would advise using a fluid filled gauge to dampen out any oscillations, as well as vibrations from the pump.Since the fluid on the gauge side of the diaphragm is incompressible, it will faithfully reproduce the pulses from the pump.A self-cleaning snubber is also appropriate in this case,before the diaphragm.

Here is a link to a supplier of teflon coated SS isolation diaphragms :

http://www.dginstruments.com.au/l-category30/pressure/isolating-diaphragm-seals/

No joy over the phone with the pressure gauge distributor, then?

lightning.arrestor-

Utilize electronics. Obtain a disc type pressure/electronic pressure sensor with a Teflon coated or Teflon disk (this is similar to the disk isolator mentioned in other answers). This senses the pressure on one side that is Teflon faced or Teflon construction and converts it to a electronic signal. For this signal use a standard converter to utilize this signal that can be read on a remote display unit.

This reduces the units in contact with the corrosive properties material to one. Snubbing for the pulses of the recip pump can be done at the indicating display by adjusting the display settings.

This is how the pressure of corrosive materials is done in the chemical industry. Also for remote reading pressures. Similar to utilizing a load cell on a weight scale with a remote display.

Good Luck, Old Salt

You can special order seal diaphragms with restriction valves to act as snubbers, what can be done though is place a syphon and then connect the seal on top of it, it should work.