By Davi Correia, Senior Mechanical Engineer
Writing valve specifications is a more humble activity than discussing nuclear treaties, but the proverb applies just the same. Information about a valve is provided, either by a maintenance team or a by design team, and a specification must be written in accordance with this information. The thing is, sometimes the information received at first does not tell the whole story.
Back to Basics
Generally speaking, the major valve standards provide specification guidelines, like Annex O of API 6D and Annex B of API 600. A common arrangement in many companies is that the valve specialist filling up these guidelines does this for a variety of plants and valve locations. Consequently, the specialist is not familiar with the many particular details of each application.
For the most part, it is not unreasonable to trust the information received, but there are times when it is advisable to stop and conduct further inquiries in order to verify that the data is sound. There is no universal telltale sign for when to do this verification. Only experience can help in this. Let us take a look at some examples of important information being withheld from the data sheet.
Example #1 Spatial Constraints
The location the valve will have in a piping system may affect its function. When people think about this, sometimes the only concern is if the valve will be installed in a vertical or a horizontal position. Although this is a valid issue, there are other matters to be attended. One of the most important is informing the supplier if there is some spatial restriction on the location the valve will be installed. For example, the supplier may need to limit the actuator dimensions in order to avoid interference with existing structures (see image above). Even if you are ordering the valve with the original supplier, you must inform of such restrictions, for the supplier may consider changing the actuator model due to economic or time factors. Spatial considerations can also affect hand operated valve models. Such is the case when the valve has a specific flow direction and the handwheel must be provided on a particular side of the valve. If not, the handwheel may interfere with other piping or a structure. It may also render the valve useless, as the handwheel may be too close to a wall for the operator to squeeze in and turn it.
Example #2 Environment
Sometimes, large industrial installations develop “micro-climates”, that is, due to wind direction or nearby processes taking place, the conditions of humidity, temperature, salt buildup, chloride content, etc., may change in some specific spot in the plant. A valve located in the same service upstream or downstream of that area may work flawlessly for years, but the valve in the spot fails in months. When this pattern is noticed, further investigation is usually required to understand the problem before changing materials or the valve design.
But first, the pattern must be noticed and it is incumbent to the operational team to give the adequate feedback on valve failures in order to mitigate the problem. One critical example of such a situation is SCC (Stress Corrosion Cracking) in austenitic materials, particularly bolts. A common valve type that is used with no problems throughout the plant may catastrophically fail in a particular location due to a micro-climate. This is more prone to happen in AISI 304 bolts, but it can also happen in other grades.
Example #3 Fittings
Some designs require valve cavity pressure relief devices. If that is the case, it is good practice to consider modifying the standard fitting supplied by the vendor. It may be worth checking if the model and the materials used are adequate for the fluids in question (inside and outside the valve).
For offshore applications, for example, it may be advantageous to specify the spring in UNS N06625, in order to avoid materials that may be prone to Stress Corrosion Cracking (SCC).
Other fittings that also requires attention are the ones related with injection points. There is no consensus in the valve world as to when to use them. Some argue that without them there is practically no options to save a leaking valve in the field, while others argue that they are only potential leak paths through the valve. One thing is sure: if you do want them, it is not enough to put a “Yes” beside the line saying “Injection points” in the data sheet. It is necessary to specify the model, the quantity and the location you want them. The recent revised S-562 provides valuable insight on this issue for ball valves.
Example #4 Process Conditions
If you are using carbon steel trunnion ball valves, it may be good idea to consider applying a Corrosion Resistant Alloy (CRA) overlay on seat pockets and stem sealing areas. That is because the seats in this valve model need to move in order to function properly. Sometimes the fluid process is not supposed to be corrosive, but there is always the possibility of a hydrostatic test or some stagnation condition during shutdown to provide a corrosive environment in this area. If lip seals are used, this is all the more critical, for they are extremely unforgiving to irregularities in the sealing surface (when compared to the traditional o’ring). Just to be clear, the advice here is to apply overlay only on the sealing areas! This is not the same, cost-wise, as to apply overlay on all the internal surface of the valve.
Example #5 Preservation
After a valve is delivered to the end user, it may be stored in a warehouse or it may be forwarded to a place near the location of installation. In the latter case, the facility may not have a covered area and the crate with the valve may be exposed to rain and sunlight for a period of time. Such is frequently the case on offshore plants, due to its lack of sufficient storage area. The standard packing procedure for the majority of valve manufacturers is not designed to withstand outdoor storage. When ordering a valve for this situation, it is good practice to discuss the matter with the supplier and see what field proven solutions can be arranged. Also, it is important to warn the maintenance team to not modify or be creative with a valve package that is already prepared for the outdoors.
These are just a few examples of situations that may arise when specifying a valve. Whether the valve is to be used in maintenance or a new project, it frequently pays to “dig deeper”. That is, to understand the application and the real conditions affecting the valve, starting with its delivery throughout its operational life. Remember, the smallest detail can become a huge money drain for just lack of a little more effort in the data sheet.
P.S. By the way, Mikhail Gorbachev only once replied to Mr. Reagan’s proverb. It was with a quote from the American philosopher Ralph Waldo Emerson “The reward of a thing well done is having done it.”