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When it comes to vessel maintenance, valves aren’t always appreciated for the vital role they play in a vessel’s functioning. In reality, it’s vital that they are properly maintained and in good working order – so much, in fact, that class society has stringent guidelines on how to care for them.
By definition, a valve is a device that controls the flow of fluid. In marine applications, there are several types of valves and they’re grouped into four main categories: linear shaft valves, rotary shaft valves, check valves, and control valves.
Linear shaft valves are labeled as such by having a closure member that operates in a straight line to regulate flow. Gate and globe valves fall under this category. A gate valve consists of a body having inlet and outlet connections, a bonnet, and a gate-like disk attached to a threaded stem. They are fitted with two or more packing rings around the valve stems compressed by a gland or nut to seal it. A disk fits between two tapered seats within the valve body to seal it closed. These valves are low resistance and don’t throttle fluids well.
Globe valves, like gate valves, are fitted with a disk that is mounted on the end of a threaded stem. The valve is closed when the stem is screwed into the bonnet, which forces the sealing surface of the disk, against a mating seat in the valve’s globular-shaped body. These valves throttle fluids well, but there is a higher resistance to flow, as the fluids passing through have to change direction within the body. Some globe valves are lever operated, angle, needle, diaphragm or spool valves.
Rotary shaft valves have a passage or passages that rotate in a transverse plug to regulate the flow of liquids. Butterfly and ball valves are two types of rotary shaft valves. A butterfly valve has an internal disk that tends to be equal the size of the connecting pipe’s diameter. The disk can be rotated from a fully closed position, which is perpendicular to the flow, or to a fully open position parallel to the flow. This can also be used to throttle the flow of water.
Ball valves consist of a spherically shaped plug with a round hole passing through it that can be moved from a full open position to a fully closed position by rotating the handle. These valves can also be found as part of three-way valves with the third port on the side or bottom of the valve body.
The sealing surfaces become exposed when trying to control the flow of fluids, making erosion a problem. For this reason, they should not be used to throttle fluid.
Check valves are used to prevent reverse flow through a line. The pressure of the fluid entering the inlet side opens the valve, and the valve is closed by the fluid entering from the discharge side. Lift check, ball check, swing check and stop check are all variations of check valves.
In lift check valves, a piston or disk raises and lowers from the pressure of the fluid coming from underneath it. Ball check valves operate in much the same fashion, but have a guide for the ball. Swing check valves have a disk mounted on a hinge that swings off the seat by pressure from the inlet of the valve. Stop check valves are similar to globe valves, except its disk is not attached to the valve stem. When the stem is screwed tightly into the valve body it prevents the check valve from opening.
The last type of valves are control valves, which are used to automatically regulate flow through a system to maintain a required pressure, temperature, flow-rate level or some other parameter. They use a spring, diaphragm or bellows to control the valve position and parameter. Some are controlled by a programmable logic controller (PLC) using electrical current or air pressure to send signals to the valve.
Regardless the valve, there are common maintenance action items required to ensure they are kept in good working order.
Valve wear and maintenance varies a lot on how the vessel is run. When the vessel sits alongside for a while, there tends to be more growth inside the through-hull valves due to less flow through the valves. Stagnant water inside ball valves leads to crevice corrosion, which can be detrimental to its sealing.
Always exercise precise care when maintaining valves. They’re much easier to maintain then replace.
Rich Merhige is owner of Advanced Mechanical Enterprises and Advanced Maintenance Engineering in Ft. Lauderdale (www.AMEsolutions.com). Comments are welcome at firstname.lastname@example.org.