4 replies to this topic

[stu]

Dear all,
when I came to read about valve types, the term "Blocked In Volume is high when the valve is closed" for certain types of valves.please explain me what is blocked in volume in valves.
Thanks in advance.
Stu

[gvdlans]

Dear all,
when I came to read about valve types, the term "Blocked In Volume is high when the valve is closed" for certain types of valves.please explain me what is blocked in volume in valves.
Thanks in advance.
Stu

For ball and plug type valves there will be a volume of liquid or gas trapped inside the ball/plug when the valve is closed. When you look at the drawing of a ball valve you will probably understand what I mean.

Drawing of ball valve

[Art Montemayor]

Stu:

You are very observant in noting that certain valves have a potential for a hazardous situation. This, in my opinion, unfortunately is something that escapes the observation of even the majority of professional engineers. This is a very important point and a valve feature that should be lost on no one in the processing industry. The types of valves that have this inherent feature (i.e., that retain a volume of fluid within themselves in the closed position) are:

1) Ball valves;
2) Plug valves;
3) Cock valves (a type of plug); and
4) Even some types of gate valves.

This is just another reason why I always insisted on all young graduate engineers serving at least one year within the maintenance department – on shift – in those processing units I operated or managed when I was in operations. This type of detailed knowledge makes for a stronger process design engineer and also for a safer engineer. Those that know me well also know that I'm known for constantly stressing that engineers always be aware of and look for what are process "trade-offs". This follows the philosophy of "no free lunches". Nothing in life is free nor are there miracle solutions to problems without having to pay a price or a trade-off.

I clearly remember the time when ball valves became a "craze" in the processing industry and they were supposed to resolve all our existing leakage, closure, mechanical, and maintenance problems in our plants. What happened, as we now sit back and review the past 50 years, is that they resolved some problems – but they also introduced others. The "solution" wasn't "free" and that easy. All ball valves, by their very nature, introduce an inherent, potential hazard into a process that – if not taken into consideration under certain circumstances – can cause great harm and danger. When turned 90 degrees into the closed position, the "bore" of the ball valve is naturally sealed between two seats. If the seats are effectively 100% sealing, the fluid that is "trapped" within the bore (and the body) of the valve will be static and subject to a lot of things:

1) the fluid (if liquid) can expand hydraulically if heated from the outside and cause tremendous pressure on the valve's body – something that is not considered in the basic valve design;
2) the fluid can decompose or polymerize within the bore and sometimes "cement" or plug the valve, sometimes preventing it from turning to the open position.
3) The fluid trapped can be a potentially contaminating fluid for subsequent downstream operations.
4) When ball valves have been used in such applications as block valves for liquid carbon dioxide, liquid propylene, liquid ammonia, and other liquids that are handled in the sub-ambient temperature state and pressurized, the liquid trapped within the bore has often expanded and caused one of the seats to rupture and fail to seat. This is an example of 1), above.

To mitigate the hazards of thermal liquid expansion, I always specifically specify that ball valves be drilled with a small hole that connects to the upstream side of the valve. Another way to alleviate thermal expansion or potential contamination is to install a small "bleed" valve into the side of the ball valve and manually drain the trapped fluid in the bore after closure. I also demand that all ball valves be visually confirmed as being protected against thermal expansion and that the correct direction of flow be confirmed when a hole is drilled into the ball. I do not employ side bleed valves because I place little faith in humans following the manual procedures 100% of the time. I prefer to have all my ball valves drilled and marked for direction of flow.

I hope this addresses this important subject to your satisfaction and benefit.

[Ali66]

Well-said Art. Thank you.
Ali

[stu]

Respected Masters,
Thanks for your clear & authentic explanations for my doubts.
Thanking You.
With Regards,
Stu