Selecting Shut-Off Valves
There are many reasons for this change:
- Cast steel retains its strength better than cast iron in the event of a fire. A cast-iron valve is more likely to crack if water is sprayed on it.
- Cast-steel valves are not as heavy as cast-iron valves.
- Cast-iron valves employ steel flanges. However, flanges can leak, where certified welds seldom do.
- The cost to install cast-steel valves is less than cast iron.
A major difference between weld-inline cast-steel valves and cast-iron valves is the valve disc and seat (figure 1). Most cast-iron valves had a lead seat and a fixed disc. The cast-steel valves had a PTFE seat attached to the disc, which could rotate on a ball bearing. The cast-iron valve lead seat was known to wear with usage and poor valve operation. The valve was closed more tightly than it should have been. As a result, the lead was eventually damaged and the valve leaked.
In the cast-steel valves, the disc seat will not rotate once it is tight -- only the stem rotates, resulting in limited wear due to rotation. Critics will argue that the PTFE seat will wear more rapidly than a lead seat.
System cleanliness is a must for reducing valve seat wear. I've seen ammonia systems that have many lead seat cast-iron valves that would not close properly because of poor operating procedures. However, in each case, the real culprit was a dirty system. The ammonia was contaminated with stuff such as rust, grinding residue, welding debris, sand and even ammonium carbonate. And erosion will occur if ammonia gas or liquid flows through the valve at high velocities.
The owner and contractor have several choices in the cast steel shut-off valve area as there are several excellent alternatives. They all have similar configurations with variations in stem materials, stem packings, discs and valve body.
There are some OSHA and EPA regulations regarding valve stems and stem packing leakage. Valve packing may seem perfect; however, a very small amount of ammonia may leak out each year. The simplest, most cost-effective way to eliminate this problem is to employ seal caps on all shut-off valves. This should stop just about any potential leak and protect the stem and packing from the atmosphere, particularly when the piping and valves are on the roof. They also eliminate any possible packing leak in that work area.
Some maintenance folks do not like valves with seal caps because they must carry a wrench around with them all the time. However, that's a small inconvenience in exchange for eliminating most, if not all, stem leaks.
Another choice that must be made is between rising and non-rising valve stems. Both are more than capable of doing the job, and often it's considered more of a situational or operation choice. But there are a lot of folks out there who will tell you it's a safety issue, too. With a rising stem valve, you can tell by just looking at it if it is open or closed. That's not the case with a non-rising stem. But as I say, both valves can do the job, and there are situations where the choice of a non-rising stem valve may be more appropriate.
Another choice is the one between inline valves and angle valves. Probably one of the best advantages angle valves have over inline valves is pressure drop. Angle valves have about half of the pressure drop of an inline valve. In my opinion, every valve in a system should be an angle valve wherever possible because it saves a pipe elbow and two welds. You can't always do it, but it's something that should be considered.
Often, large shut-off valves are difficult to open because the downstream pressure is greater than the upstream pressure. This problem can be resolved by installing a small bypass valve that can be opened to equalize the pressure.
Smaller shut-off valves often have screw-in bonnets. I may not make some friends with this comment, but I'm not a fan of them. You really need to know what you are doing if you a working on one of these valves. It's my opinion that it can be dangerous. When these valves are being re-packed, the packing nut must be removed. There have been several major incidents where the entire bonnet is removed along with the packing nut. In my opinion, the safest procedure is to use bolted bonnet shut-off valves of all sizes in an ammonia system.
Whenever possible, an inline cast-steel globe valve should be installed in a horizontal position with the direction of flow against the seat. Many valves have a directional arrow on them to help indicate the direction the ammonia should flow through it. This is important for a couple of reasons. Positioning a valve horizontally prevents liquid ammonia from damning up inside the valve. It also will help keep trash and dirt from collecting in the valve stem.
The mechanical integrity of shut-off valves in any ammonia refrigeration system is directly related to the cleanliness of the system. During installation and commissioning, clean piping practices should be observed. Strainers should be kept clean to remove any trash. And, particularly on older systems, I would like to see liquid ammonia filters (coalescing-type) installed.
The IIAR Ammonia Refrigeration Piping Handbook contains a lot of good information on piping and valves. If it's not on your book shelf, it ought to be.