Something can look like a valve but that doesn't mean it is one
5 Oct 2018
Ken Roberts of Satron Instruments returns to the subject of field instrument installations with a look at the practicalities involved
A couple of months back I had my rant published about how industry sticks to the past like today just hasn’t happened. I went on about the all too common – blocked impulse lines – negative effect of the ‘that’s the way we’ve always done it’ attitude to field instrument installations.
I thought it was dead clever to point out that one of the biggest problems with the reliability of field instruments need not exist with just the use of a simple valve. Then I realised that my earlier rant was without a proper description of how it all works. How you make a safe and readily maintainable instrument installation without using easily blocked impulse lines and manifold valves and still be able to easily isolate the instrument from the process.
That’s the point, if you must stick the measuring sensor right inside your process when it breaks or needs maintenance, you’re stuck until the next plant shutdown.
Or are you?
No, you’re not. One way to make the installation maintainable is to use a simple ball valve that’s big enough to pass the sensor head through and fix the instrument to the outlet.
It is a sort of fitting just as valves are really a type of fitting but with a specific function – controlling flow
Using an elongated circular section sensor head fitted with sliding seal will allow the sensor head to be withdrawn from the process though the ball valve without leaking. Then the valve can be closed and the sensor removed from the plant for maintenance.
This method can be good enough for use on low-pressure and benign fluids but there is a better way. A way that works for almost all pressures and fluids even at high pressure. A way to insert the sensor into the process while allowing it to be safely and easily withdrawn against process pressure for any maintenance that might be required.
That way is a device called PASVE. It allows a standard G1 or 45mm diameter sensor head to be fixed to the bore of the ball and rotated and slid forward when needed while a simple part turn isolates the sensor using double block and bleed for safety.
Mechanically, it’s a bit like a trunnion-mounted ball valve but without a pressure retaining body around the ball. The ball has the sensor head mounted in it using a standard threaded G1 or similar connection with the seal being either a tapered cone or ‘O’ ring in a groove.
The ball itself is sealed using two separate ball valve seats with a gap between them giving double block and bleed isolation of the process fluid when required. The instrument is supported by the device making the whole installation one simple tightening of a G1 nut and the electrical connections – that’s it.
I’m not sure you’d call PASVE a valve; it doesn’t control the flow of any fluid it just allows the passage of a sensor. It is a sort of fitting, just as valves are really a type of fitting but with a specific function – controlling flow. Is PASVE a valve? It looks quite like a valve and it operates like a valve – but is it really a valve?