Globe Valves Used Aboard Ships - Operation, Design, and Repair

A globe valve is so named because of its globular appearance and because some valves of the past had hemispherical shaped valve plates inside. However today globe valves with the shape of a circularly cut flat valve plate to hemi spherical shaped valve plate are available in the market that do not conform to the traditional old design. The shape of the valve plate depends on the application, the manufacturer’s design, and the control required. Globe valves are used extensively in industry and very frequently in the marine field. They are very popular due to the following characteristics:

• They have a long service life.
• They can control the flow of the fluid passing (though not much, and for a finer degree of control, control valves with special valve profile must be used).
• They can be kept open any degree from full open to full close.
• Very little valve lift is required to attain full flow (unlike gate valves).
• A change in direction of the fluid flow can be done (unlike gate valves) in angular valves.

In a globe valve the fluid to be controlled always flows from below the valve seat, so as not to pressurize the stuffing box and the gland packings. The globe valves are used for specific applications where stringent shutoff requirements exist. They normally use metal-to-metal seating surfaces, but sometimes use soft-seating like PTFE or other composition materials forming the sealing boundary.

This article is important to marine engineers and is in the syllabus of MEO (Marine Engineer Officer) Class IV and Class II exams. It covers some portion of the function "Marine Engineering Practice". This question is a favourite in the written and the oral examinations .

• Wheel nut: This is the nut at the top of the wheel and it helps to retain the wheel on the spindle or the stem. It often has a plain washer beneath it.
• Hand wheel: It is provided to open and close the valve. The bigger the wheel, the less effort is required to operate the valve as the torque applied is more. This is the reason wheel spanners are used on board ships, as the longer the arm, the more the torque.
• Yoke nut: It has internal threads and the stem or the spindle of the valve has corresponding external threads. When the spindle is turned by the wheel handle, the stem rises or falls thus opening or closing the valve.
• Gland bolt: These are used to tighten the gland flange and the gland bush, thus in turn compressing the gland packing and stopping leakage along the stem of the valve. In ships generally a stud with nuts are used as they are easier to access and maintain.
• Gland flange and gland bush: They are provided to compress and keep the gland packing in place. Sometimes they are combined together and form one piece.
• Gland Packing: They are cut as per the size of the stem and put end to end, there should be no gap between the ends otherwise the fluid may leak out. The packings are staggered so that the cut portion may not lie over each other and provide an easy channel to pass through. Several packing material can be used depending on the applications like PTFE, graphite ribbons, etc.

• Bonnet: This is the top portion of the valve and it has an opening for the stem to pass through.
• Bonnet bolt: The bolts are used to tighten the bonnet against the body and compress the gasket in between, thus effectively sealing against leakage.
• Back seat: removable and replaceable part for maintaining clearances, and guiding the stem, and reducing leakage
• Gasket: It is made of a compressible, oil proof, chemical proof, and heat proof material and put in between the bonnet and the valve body, to stop leakage along the metal faces.
• Stem: It is also called the valve spindle and is connected at one end to the valve seat (in a screw lift valve) and other side has the wheel. It is threaded on the upper part.
• Disc: Also called the valve disc, its lower end is correctly machined and sits on the valve seat. The valve and the valve seat have face to face contact and must be true to each other to stop the flow.
• Seat: Also called the valve seat, it is sometimes a replaceable insert
• Body: Also called the valve body, it is the housing.