Pre-evaluating before maintenance

One must evaluate the media: Is it a clean gas, or is it heavy hydrocarbons with a lot of pollution? Is there an aggressive or sour media, or is it soft? Then the construction of the valve: Is it selfrelief or double piston? Is the valve soft seated with soft PTFE or is it a harder material such as PEEK? Or is it metal to metal with a tungsten carbide layer on the ball and seats? What type of radial seals are there on the seats? Are they O-rings, chevrons or lip seals? How is the valve operated? Is it opened with differential pressure, or fully equalized?

Is the cavity equalized or not? Has the valve been closed with full flow through the valve? How often is the valve operated – once a week or once a year? How fast is the valve operated? Then the two most important questions; is the valve equipped with lubrication fittings to the seat, if so, how many to each seat? Is the valve equipped with an auxiliary valve to the cavity? If the valve is equipped with fittings and an auxiliary valve there are (as we have seen from previous articles) a lot of possibilities for maintenance, and in many cases the valve may be 100% cured, but in other cases it may be worse after cleaning, it is not the maintenance that has damaged the valve, but the maintenance has uncovered the state of the valve.

Jumping to conclusions

In this article, I will discuss two cases occurring on two large valves, one where I was misled by my own assessment and one where I was lucky when performing the fix. I am using those cases to illustrate how one can be fooled into jumping to conclusions, or how one can be lucky when doing the right assessment. The first example involved a top entry trunnion ball valve containing 140 bar clean sales gas with condensate.

In this case the valve was needed as a first barrier and should have been 100% sealed, but the valve was leaking too much. The valve information was as follows: Self-relief construction on the seats which sealed on the upstream seat only. As illustrated in Figure 2 the valve was equipped with soft-seated seats with PTFE inserts. As also illustrated in Figure 2 the radial seals on the seats were O-rings. The valve was normally in the open position, operated only one or two times a year and had been in service for about 10 years. The valve was never closed in flowing condition or opened with differential pressure. At the assessment phase there seemed to be a simple solution on the problem. My argumentation was as follows: not opened with differential pressure, not closed at a flowing condition and maximum operations 20 closures and 20 openings equals 40 of 90 degrees movement. With so few operations in a clean media and operating with a fully equalized valve I could not see any reason why the PTFE seal on the seats should leak. As the media was clean sales gas I did not think that deposits could be formed on the ball or seats.

That argument was also based upon the fact that the condensate should actually work as a cleaner on the seal surfaces. My conclusion to this problem was that the O-ring on the radial seal of the seats may have suffered explosive decompression, or RGD, and the leak path was not between the ball and the seat but between the seat and the seat pocket, as illustrated in Figure 2. This could be fixed by injecting sealing component into the O-ring groove.

Fooled by the valve

Because the conclusion in the pre-assessment was RGD in the radial O-ring we did not clean the valve with valve cleaner, but regarded that as a waste of time and valve cleaner. To seal off the O-ring we injected a fair amount of sealing component, I even thought it was too much required to seal off the valve. We were unable to seal the valve 100% but we managed to get the valve nearly sealed off and kept the cavity without pressure by opening the auxiliary valve in the cavity. We managed to do the required job, but because of the relatively large leak rate passing the valve it was decided to repair the valve in the line (top entry valve). The whole system was shut down and depressurised to dismantle the ball and seats.

When the seats came out, I could see that this time I had been fooled by a valve. It had tricked me as the O-rings were in perfect condition. But, as illustrated in Figure 3, the seal area on the seats was full of hard particles that were stuck on the seat. The particles were some 10th of a millimetre high and prevented the seat from coming into contact with the ball. We found the same black stuff in the spring pockets and on the coil springs, illustrated in Figure 1.

As the media was clean gas and condensate I was quite puzzled. I did clean the seats with valve cleaner and, as illustrated in Figure 4, the particles on the seal area were dissolved. It was decided to replace all internal parts, and the particles were sent to a laboratory for analysis. The result was: Under certain conditions (temperature and pressure drop) the gas molecules will bind in chains and transfer from gas to solid carbon and bind to the seal. I have never seen this before, but it taught me an important lesson: If the valve has been without maintenance, always clean it. It doesn’t matter if the media has been clean. If we had cleaned the valve it would have been fixed and probably sealed 100%.