All parts that come in contact with the media must be able to withstand the impact from that media preventing corrosion on internal parts witch could create a leak or damage to the valve.
There is yet one part that sometimes “drops out” of this material selection and causing problems: the pressure gland. This is the part that compresses the stem seals from the outside. On a gate valve, it is the light blue part on illustration 1, and the light blue part in the middle of illustration 2 is a similar part on a ball valve.
If the body/bonnet and stem are made of 316, it is a good idea to select a material of another hardness then the one on the stem. This to reduce the danger of galling when rotating/ lifting. As long as the press gland is not in contact with the media, carbon steel will in many cases be selected. As long as the gear/ actuator is made of carbon steel, there should not be a problem selecting carbon steel on the press gland. But there is one drawback on this selection, namely water from the outside. This may be condensation slowly creeping down or rainwater going down the stem.
In 2015 I went to one of the offshore installations on the Norwegian sector to try and solve a challenge they had with eight ball valves. They were 6” class 1500 manual valves made of 316. The problem: the valves were very hard to operate and had been in service for around six years. To be able to operate the valves, they had to use big cheater bars. Before I came out, there had been a serious discussion to order new valves, but as most of you know, the delivery time was around nine months, and they decided to forward the challenge to me.
When I inspected the valves, they all looked fine with a nice white paint coating. The valves were under a roof out of the rain, and they regularly cleaned the area, hosing it down with water.
As I always state: analyse the situation before doing anything!
First, what can induce increased friction on a manual ball valve? Three things: gear, stem and seat to ball friction. To check the gear, we disconnected and operated it. In other words, the gear was fine! To control the stem friction towards the pressure gland, the gear connection plate was disconnected and taken off. This could all be done with the valve fully pressurised as we knew from the drawing that the gland plate was bolted to the top of the valve. When the gland plate came off, we clearly could see the problem, as you can see in illustration 3. The gland plate was produced from carbon steel, and when hosing down the area, the water seeped down to the plate causing corrosion between the plate and the stem.
As you can see in the third illustration, you don’t have to be a rocket scientist to solve the problem. Now the pressure had to be reduced to atmospheric as we had to take of the gland plate. After removing the layers affected by corrosion, reinstalling the plate and the actuator, the valve was just fine. We did the same with all the eight valves, but we improved the situation by filling the gland plate area with grease and sealing off the top of the gear. By doing so, we prevented water seeping down. Years later, the valves still work perfectly. Who says maintenance doesn’t pay off?
About the expert
Norwegian consultant and valve instructor Mr. Ingolf Fra Holmslet writes a series of articles for Valve World.
Klyde Consultants AS