Valves in Severe service
This issue of Valve World magazine are related to severe service, and that put a question in my head: What is severe service?
If that question were asked, I think most would probably answer: Sour service, sandy service, high velocity or cryogenic, to mention some. And yes that is examples of severe service, but could low-pressure water be severe service? The answer is: Yes it could. In my head any media could be severe service if the valve don´t cope with it. For some people running a marathon is no problem, but for most it would be impossible (without a proper training and preparation for the event). Ok let’s put that into the valve world context: If the valve is not prepared for the task it will be installed in; it would in my mind be severe service for the valve.
A rising stem ball valve class 900 was installed in produced water with some sand. The valve was working for years without problems under non-severe conditions. Then one day the valve was uses as a throttling valve for some hours and came under high DP conditions. The result; take a look at fig. 1 the ball, fig. 2 the internal of the valve and fig. 3 the outside of the body
When throttling the valve, it came under extreme conditions due to the use, or abuse of the valve. As this valve is ok for opening with DP or closing in flowing condition it is a fare stretch to use it as a throttling valve with 80 bar DP with two-phase and some sand. This was a standard condition that vent severe due to improper knowledge from the operator or the person ordering that operation. The result of this was heavy external leak with oil and water and a shut down of the installation. Total cost: Fare to much, but no personal damage.
Figure 4 is a conventional trunnion ball valve with soft seat insert. The valve was in gas service with a system-pressure of 130 bar, material 316 and PTFE insert in the seat as illustrated. Nothing severe until the operator decided to open the valve with 100% DP, and as one can understand he did not fully open the valve at once due to down stream equipment. But as any good operator would do; carefully opened the valve to pressure up on the down stream side, and again; a standard valve, in a severe condition for that valve. One should be carful to open a soft-seated ball valve with DP and especially with 130 bar. There is a rule of thumbs: You should not open a soft seated valve with a DP higher the 20 bar. 20 bar can in fact be to much if the valves is put into a 15% open position for e longer period. A standard metal/ metal-seated hard faced ball or gate valve should not be opened with a DP above 60 bar. Of cause it depend on the media, the time used for throttling —–, but again this is my rule of thumbs.
Fig. 5 and 6 illustrates what normally are considered a sour service, but the situation should not give the result illustrated. The valve was a revers operated solid slab with material 316 and Graphite + PTFE stem seal, the media was produced water.
As the valve was a normally in open position (reverse operated) and as we know in open position the stem stops in contact with the bonnet seat isolating the volume in the stem seal area. As the valve is in that position for month at the time the media trapped above the stem/bonnet seal goes sour and H2S is formed, attacking the stem. The bad gay here are the graphite: Bad combination; water and graphite. Again the media was in fact NOT severe, but it all did end up in a stem leak. In this history it was not one valve, but several.
We could go on with hundreds of samples, but I will end with one more, a standard globe valve with should take regulating, but it did not. The high DP eroded the seats and gave a heavy leak. The thing is, this valve was sub-sea and not easily replaced, and nobody thought it was necessary to install a valve with TC plug. After all it is much more expensive.
Who is to say when the valve ends up in a severe condition? If ball valves are without lubrication fittings one cannot clean the valve, witch again could lead to seats stuck in the seat pocket resulting in a leaky valve, witch must be replaced as there are no possibility for valve cleaning. Lets take a ball or gate valve, without lubrication fitting to the stem. Normally OK, but the day you do suffer a stem leak you can´t inject sealing component to seal of the valve witch would have solved the situation.
As most readers of my article probably know: I am all in for maintenance of valves! This goes for valves in severe services too. Many tend to skip the fittings as that is a potential leak point but without those fittings; no maintenance possible. I do understand that in some cases like at a raw oil cooking in a refinery with 560 degree C, you don´t install fittings and add leak points as the media is self igniting, you DON’T want a external leak, period. Back to my marathon runner, you may train yourself to become a marathon runner or a ultra marathon runner, but you can’t train a valve to become a severe service valve.
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