Parts 1 and 2 of this this primer discuss three common types of flow isolation and control valves used in the cryogenic industry. Part 1, below, will focus on globe and gate valves, their basic components, how they work, and their applications. Part 2, in next month’s issue, will cover the same topics for ball valves as well as overall design considerations, materials of contruction, components and features, installation considerations, and general maintenance.
Cryogenic flow isolation and control valves
Valves are used all along the industrial gas and cryogenic gas supply chains – from production and transportation to trans-fill operations into storage containers. Three categories of valves used are: 1. valves that isolate product flow; 2. valves that control the flow of a gas or liquid, and 3. valves that regulate pressure or flow, or relieve excess pressure. These include globe, gate, ball, butterfly, pressure relief, diverter, needle, check, and diaphragm valves, and regulators.
Merriam-Webster defines the word valve as, “Any of numerous mechanical devices by which the flow of liquid, gas, or loose material in bulk may be started, stopped, or regulated by a movable part that opens, shuts, or partially obstructs one or more ports or passageways.” The word valve was borrowed from the medieval Latin word valva, which referred to ‘a bodily structure (such as the mitral valve) that closes temporarily a passage or orifice or permits movement of fluid in one direction only.’
Before diving into a detailed understanding of valves, two basic fluid principles need to be explained. The first is to understand that flow through a valve is always from higher to lower pressure.
The second is flow restriction or pressure drop. Flow coefficient (CV) is a standardized method to compare and size piping components such as valves. CV is a relative efficiency measurement of pressure drop across a fully opened valve, pipe, fitting, or orifice at a corresponding flow rate.
Technically it is defined as the volume of US gallons of 60 °F water that can flow per minute through the component with a differential pressure drop of 1 psi from the inlet to the outlet of the valve. A higher CV means there is a lower pressure drop across the valve, which equates to a higher flow rate and lower energy loss.
In layman’s terms, it is a value indicating the amount of flow restriction (pressure drop) through a piping system or through a component; a higher CV value is better. The flow through gate and ball valves produce a streamlined flow, yielding CV higher values for a given size, than through a globe valve.
The table above compares flow restriction through different size globe and ball valves. Higher CV values indicate lower flow restriction / less pressure drop across a valve. Minimizing pressure drop across a piping system is often an important design consideration.
Globe valves, named for their generally spherical body shape, with the two halves of the body being separated by an internal baffle, are perhaps the most common valves used in the industrial gas market. These valves are generally used for flow isolation and control, for both cryogenic liquid and vapor. Globe valves are used in the majority of cryogenic applications where long-term isolation/shut-off reliability is required. Cryogenic globe valves are used on fill and use lines on storage tanks, transport trailers, customer installations, and isolating services for end-of-line and maintenance requirements.
The hose bibb or water spigot on the outside of your house is a globe valve. Cryogenic globe valves are essentially of the same basic design. In the globe valve shown in Figure 1, liquid enters the left or inlet side of the valve, then flows upward through the round seat opening, around the plug/disk, and out the right side of the valve. The internal flow path within the body creates greater flow resistance than occurs with gate and ball valves.
When the globe valve is closed, as shown in Figures 2, liquid enters the left or inlet side of the valve, but is stopped beneath the disk, or plug, which has been screwed down to close off and seal against the round seat machined into the valve body. Notice that the valve stem packings are not exposed to the flow media or upstream pressure when the valve is closed, as can be seen in Figure 2. However, the packings are exposed to product and pressure if the valve is incorrectly installed backwards, so that flow is into the top of the valve, as shown in Figure 3.
Since the valve disc moves directly away from its mating seat when the globe valve is opened, it is less prone to seat damage or wear than gate or ball valves. Globe valves are, therefore, considered more reliable for long term sealing performance.
Besides isolation, globe valves are also used for fine flow control applications. They are more suited for regulating, or throttling the flow, than gate or ball valves, i.e. to control the flow being discharged from centrifugal cryogenic pumps.
Globe valve flow direction
It is crucial that globe valves be installed taking into consideration the directional flow arrow cast into the outside of the valve body, and the application where the globe valve will be used.
If you look into the inlet end of a globe valve (See Figure 4) you will see that the product enters into the valve, flows upward through the seat area, and then out the exit side of the valve. However, do not confuse this arrow direction as being the direction the valve must be oriented in a given circuit with regard to the actual directional flow of the product. More often, the flow direction of a valve should be based on the application and its criticality. In some circuits, for example the Fill and Drain on a cryogenic tank, operational flow through the valve can be in both directions. What is most important is not the product flow direction but the isolation function of the valve.
The directional arrow on a Fill and Drain valve, or on a Recirculation valve from a pump back into a cryogenic vessel, should point away from the tank in both applications. The direction the valve is installed should not be based solely on a preferred direction of product flow when filling or using product from a tank, nor be based on the flow direction when cooling down a pump, catching prime and recirculating. When installing a globe valve on a storage tank, it is critical to ensure that the liquid remains below the disk (plug) and seat when the valve is closed (arrow pointing away from the tank, as shown in Figure 2). In this case, even if someone were to intentionally loosen the valve stem packings on the valve, the product from the tank would not leak. It would remain trapped below the seat.
Always consider isolating the product below the seat when installing a globe valve. Often the operational use flow direction does not matter as much as securely isolating the product upstream.
Besides being used for flow isolation, globe valves are also used for flow control applications. They are more suited for regulating, or ‘throttling’ flow than gate or ball valves, i.e., to control the flow being discharged from centrifugal cryogenic pumps. Of all the common cryogenic valve types, globe valves are considered to be the most reliable for flow isolation and control.
Immediately upon fully opening a cryogenic globe or gate valve, close it back a fraction of a turn (e.g. 1/8). This will prevent it from freezing in the fully opened position.
Gate valves are so called as a ‘wedge-shaped’ gate is employed in the design to isolate the flow. The wedge is oriented perpendicular to the flow path. When the gate valve is raised (open), a clear unimpeded flow path provides better flow characteristics than are attainable through globe valves. Full bore diameter gate valves are also available.
As shown in Figure 5, when a gate valve is closed, the tapered gate, or male-shaped wedge, is lowered down to close tightly against the female wedge-shaped seating surface along the sides and bottom of the valve body. The interfering angle of the wedge design aids in sealing off a closed gate valve. The directional arrow cast into the valve body should be aligned so that any upstream product ‘pushes’ against the wedge gate when the valve is closed. This pressure further aids in sealing off the face of the wedge. Nevertheless, gate valves are not generally used in applications where leakage at shut-off over a long service life is critical. Also, due to the sliding action of the wedge-shaped gate, wear of the seal and seat can occur if particulate contamination is present in the line.
A common application of a gate valve is on pump inlet/suction piping circuits, where flow restriction or turbulence could cause pump cavitation. Gate valves are also used on some cryogenic transport trailers, where unrestricted flow is required to reduce fill and off-load times.
The cryogenic industry could not exist without valves. The next time you open the hose bib to water your flowers, adjust the flow, or close the globe valve, remember it is very much like a cryogenic valve. Open it! Adjust it! Close it!
The author gratefully acknowledges assistance with this article from Carlos Arevalo – RegO®; Frank Magurno - Habonim Industrial Valves & Actuators – North America; and Keith Stewart, and Mario Esche – HEROSE Group.
About the author
Keith Hall is a member of the gasworld editorial board and Executive Vice-President of Engineering and Product Development at Premier Cryogenic Services in La Porte (Houston), Texas. PCS inspects, repairs, and rehabilitates all types of cryogenic equipment, and is the largest such service provider for liquid hydrogen trailers and ISO containers in North America. PCS also sells rehabilitated cryogenic transportation equipment, as well as new CO2 transports. The content herein is limited to the author’s experience and may contain errors. You should never work with cryogens based solely on this article. Always follow your company’s guidelines and never do anything you are not properly trained and prepared for. The views expressed in this article are those of the author and do not necessarily express the views of his employer.