Valves are used in order to restrict or control the flow of any fluid through a pipeline. There are different types of valves available in the market. Valves usually work by lowering, raising or rotating a disc over a seating surface. It can also work by controlling the movement of a ball over the inlet of the valve.Valves have flanges or screwed ends in order to connect them to the pipes. Since pipelines are an integral part of all types of ships, so are the valves
In this article we will learn about one of the types of valve known as Globe Valve
Construction & Globe Valve Diagram
Globe valve has a bulb like body with a valve seat, which can have a screw-down plug or a disc acting as valve. The surfaces of valve and valve seat are made up a hard alloy of cobalt and chromium with cobalt as the principal ingredient. The seat is secured into the valve chest with a help of light interference fit and grub screw. A spindle or stem with square or vee threads above and below the stuffing box is used. The spindle is held in the valve disc by a nut. Stuffing box is fixed with an appropriate gland around the spindle in order to prevent leakage.
Not all types of globe valves can handle the change in direction of the fluid flow. The globe valve that allows a reverse flow is known as an angle valve. In the reverse flow, the flow is from under the valve seat and thus the valve is not subjected to higher static pressure when the valve is closed. The disc is guided on the seat with the help of the piston or a stem which is attached to a wheel handle outside the valve.
Types of Globe Valves
Globe valves are distributed into two types on the basis of the disc attached to the spindle. They are:
- Screw-lift type - If the disc is attached to the spindle
- Screw down non return valve - If the disc is not attached to the spindle.
The Screw Down non return valves are generally used in bilge systems to prevent backflooding,as feed check valves or as boiler stop valves. In both the types of valves a guiding vane or guides are required in order to keep the valve concentric with the seat when it is in the open position. The greatest lift that a valve can achieve is one-quater of a bore. For this reason it is essential that the stem is of a length greater than that of the lift.
Non return valves or the free lifting valves are generally fitted in the compartments having bilge suction lines. They are used to prevent flooding in case of damage due to collision.
A normal globe valve is operated with the help of a handwheel which is attached to the stem. Anti-clockwise rotation of the handwheel moves the stem over the threads, lifting the valve from its seat and thus opens the valve. Clockwise rotation results in the reverse process.
In a non return valve, there is no stem attached to the valve. The valve opens and closes according to the pressure of the fluid. When the pressure of the fluid becomes greater than the pressure of the valve, the valve lifts off its seat. There is no reverse flow allowed in this type of valve.
Reference & Image Credits
Marine Auxiliary Machinery 7th edition,1995 by McGeorge