Getting snubbed doesn’t have to be a bad thing. In fact, something called a “snubber” may be just what you need if you are measuring pressure in a system where pressure spikes and pulsation are a concern.
PRESSURE SNUBBERS AS A SOLUTION
In the past week, two customers have called asking for a solution to processes damaging their gauges.
One had a gauge connected to a line downstream from a quick open/shutoff valve. When the quick opening valve was activated, the rapid change in pressure caused an internal seal of their pressure gauge to rupture, allowing water into the gauge casing.
An up close view of the porous disc type of snubber shows the sintered disc that filters and slows fluids and gasses.
The gauge was rated for the nominal pressure the customer was measuring, but the pressure spike pushed past that pressure range just enough to break the gauge and make it unreadable.
It happened with two makes of the gauge from separate vendors, so the customer knew it wasn’t faulty manufacturing.
We recommended the customer look into a snubber, a specialty part with a few different design types which work to arrest rapid pressure fluctuation and spikes by slowing the maximum speed a substance can travel.
Snubbers also make gauges measuring quickly oscillating media more readable and less likely to suffer damage.
Those familiar with plumbing might equate them to water hammer arrestors installed in homes to lengthen the life of the pipes.
TYPES OF SNUBBERS
On the low-cost end of the snubber spectrum is the porous disc type (also known as a “filter type”). The fixed disc creates a semi-permeable barrier between the incoming pressure and the pressure gauge’s orifice. The pressure meets that disc and has its force distributed through the metal mesh, slowing it so that it will not harm the gauge. The pressure rises gradually, so that the gauge doesn’t jump a gear or burst a bourdon tube.
The porous disc design allows consistent pressure at low speeds, though, so that the gauge’s readings are not affected by the filtering material.
The downside of a porous disc snubber is that it can become clogged, depending on the process being measured. In that case, the readings would be affected and the snubber would have to be cleaned by flushing it from the gauge side with a solvent.
A piston-type snubber has a little more advanced design that is often self-cleaning. Often designed in two pieces, the piston inside the snubber moves freely, acting as a barrier to the rapid increase in pressure. When pressure increases too rapidly, the piston is forced against the orifice leading to the gauge, stemming the flow for milliseconds. As long as it is just a spike, that should be long enough to avoid damage to the gauge.
This type of snubber can usually be “tuned” to meet a process’s needs by using easily changed pistons of varying sizes. The diameter of the piston and its clearance within the snubber determine the rate of “dampening” that occurs.
A third “adjustable” snubber takes that fine tuning to another level. Many use a combination of a ball check to block surges and a kind of choke valve to smooth out the flow of material into the gauge. The ball check acts a lot like the piston-type snubber in that it is a passive element until a rapid change in pressure pushes it into action.
Even adjustable snubbers without the ball check can be effective in smoothing out a lot of pulsation, thanks to the integrated choke valve, which will only allow so much fluid through at one time. In a pinch, a regular needle valve could be substituted, but that is not necessarily a permanent solution to the above customer’s problem. In that case, it was a major momentary spike in pressure that damaged the gauge and a needle valve alone wouldn’t have completely arrested the surge.
Many adjustable snubbers also have the benefit of leak-tight shutoff, allowing the operator to remove the gauge for repair or removal.
Pressure ratings for snubbers can range from 1,000-20,000 psi, depending on type, size and the material used. Be sure to consider how much pressure might be introduced through spikes, water hammers and pulsation, as well as the media being measured, when deciding on the right snubber.
Finding the right one can improve gauge readability and protect the gauge from serious damage.