Basics of industrial final control elements 3 (Updated)

Discussing control valves, we have seen that the valve plug has an important part in deciding the control of flow. But one more thing, before we move on. What if we need more flow from the same valve? One way is to increase the size of the seat, and invariably the size of the plug as well, but that will need a bigger body? wouldn't it. What if the space constraints prohibit a bigger valve installation or some other restrictions are there?Here the concept of double seated valve comes in where instead of a single plug and seat, dual plugs mounted on the same stem are used with two seats. Since the flow from inlet to outlet passes through the seat opening, having two seats rather than one will give the flow dual paths hence the flow through a control valve can be increased.

A single seat/plug arrangement verses a double seat and plug. What do we have here now? two paths for fluid to pass in a double seated valve rather than a single one. But have you noticed something in the double seated valve? Yes... The second plug is not the same size as the first one. Here we are not playing find the ten differences, but one more thing worth noticing is that the TRIM of both plugs remain the same. It is not that the upper plug is a quick opening and second one an equal percentage. The reason for the first, which is having a smaller plug in the bottom and so the seat as well, is because the second plug should be able to pass through the first seat. If both plugs are the same size, the installation will be impractical, unless we cut the valve in two, put the plugs inside and weld again.... Here we should remember what we have mentioned about the body in our last article... It is normally casted out of a mold and comes in one piece (in most cases). There are some valves, specially ball valves having two body parts bolted together. But generally speaking in industry we find cast bodied control valves in abundance. The reason for the second, which is having same trim is to keep the valve characteristics uniform. So in the above case we can not double the flow as second seat is smaller but can increase the flow to about eighty percent roughly. The most generic disadvantage of a double seated valve is that it will be having more leakage around the seat, as metal to metal seats are normally having a passing tendency and since here we have two sets so double trouble. Double seated valves are recommended where leakage class (another sub topic discussed below) is not a critical issue in the control loop, like in some flow and level applications. But that's not all folks. In some applications we have triple seats and plug arrangement. That is most of the time used in high pressure applications with an ANGLE VALVE. We will be taking up this topic later

Tip...tip...tip, what is this sound? Oh water has started leaking from the washbasin faucet. Lets repair or replace it. Simple valve fault and a simple solution, but in industry where sometimes the fluid being controlled is at high pressure, or at high temperature, or both or toxic, or flammable or corrosive, or radioactive or. or. .or... , the leakages can result in severe loss of lives or become a health hazard or can damage costly equipment and systems if the control valve leaks or fail to control the flow.How to reduce the leaks in a metal to metal seat? In a single, or even double seated valves, though plugs and seats are precisely machined, but some tolerance remain when these are assembled together. The leakage further can be reduced when the seat and plugs are lapped together before final assembly. This is done by using a grinding paste of different coarseness based on the material and application which is applied between plug(s) and seat(s) and the stem is rotated to reduce the clearance between the two. Maintenance and valve repair professionals are well aware of the importance of lapping a seat and plug. This method is also used when a valve in service has developed more passing that normally allowed, to repair and reduce the leak.

The leakage in a control valve can be controlled by using some soft metals or sometimes Teflon based seats or other materials which reduce the passing. Plugs have graphite or Teflon rings when used with a caged seat which further reduces the leaks. The passing is critical specially in pressure control and also in other process parameters, so any control valve being selected should be evaluated if it meets the leakage classification required by the process.How do we normally check if the valve is having any leak (passing) fluids between seat and plug. There is another leak in the valve which is through the stem, and here we are not discussing that leak. The manufacturers have tests and define the valve leakage class before it is marketed, so the buyers and process engineers can decide which valve to use for a particular process based on its leakage class. Standards have been developed specially ANSI standard, which assign leakage class for different leakage rates from class I to class VI where class I leakage is the least stringent while class VI is the most stringent of these classifications. The amount of leakage is calculated based on the full flow capacity of any valve.

A valve having leakage Class I which is the most lenient one does not performs any specific test and is termed as dust tight. Valves we use normally in buildings and domestic application falls under this class.

Class II leakage is defined as valves have no more than 0.5% leakage of full flow at an inlet pressure of 50 PSI (340 kPa) or less if operating pressure is lesser, of AIR pressure applied at the inlet at normal operating temperature of the valve.

Class III leakage of valve states that the leakage under the above (class II) operating conditions should not be more than 0.1% of the full flow. From here and in other classes, lapping and use of soft seats are used to meet the standard.

Class IV requires seal rings of graphite or Teflon with soft seats as the leakage from this class of valves under above operating conditions should not exceed 0.01% of full scale.

Class V valves leak of less than 5×10?12 cubic metres, per second, per bar of pressure differential, per millimeter of port diameter, of water when tested at the service pressure is the standard.

Class VI valves are slightly different in that they are required [at 50 psi (340 kPa) or operating pressure, whichever is less] to have less than a specified leakage rate in milliliters of air per minute based on the valve size. Here the test medium is air or nitrogen while in class V water is the medium for testing. Refer to the table for details below. For lesser sizes between 1 to 2 inches 1-4 bubbles per minute is the leakage permissible. For more details ANSI standards set must be referred (below table follows ANSI FCI 70-2)Table from Gemco valves.

Once we have gone through what leakage classes are we can select the correct valve type accordingly which is suitable for that specific application. Please note that in a process or plant many different categories of leakage classes may be in use. As each loop and control strategy may have different requirements. This topic discussed here emphasizes the importance of selection of control valve based on their leakage class. Now we will further delve into the control valve types in our future articles where we will detail some of the terms and valve parts used here like a valve cage, soft seating and seal rings in addition to other details about control valves

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