Globe valve control the flow by the linear motion of disk, they are used in many industries, and it's vital to learn about the valve before using it.
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There are several basic components of a Globe valve.
Bonnet provides a leakproof closure for the globe valve body. Globe valve can have two types of bonnets. Screw-in Bonnet; is a simple type of bonnet bolted to the body with high bolting torque. It provides a durable and pressure-tight seal. The other one is Union Bonnet; it is suitable for places where frequent cleaning and inspecting is done.
The part that surrounds the plug is called the cage. It is present inside the valve's body and significantly influences the flow through the valve. As the plug or disk is moved, more part of the cage is exposed, and the flow will increase.
The stem is the component that joins the handwheel to the disk/plug. It transfers the force from the handwheel to the disc, which consequently controls the flow. Stem is a smooth rod with threaded ends to move up and down with the rotation of the handwheel.
The disk is the part that sits on the seat and stops the flow. Disk is attached to the stem and moves with the rotation of the handwheel. There are two types of plugs. Unbalanced plugs are solid and typically used in small globe valves with low-pressure drop. While the balanced plug has holes, it is easy to shut this plug because we don't have to fight with static force.
The last part is the seat; it provides a stable surface for the disk to shut off. It is either screwed or integrated into the body.
The working of the globe valve is straightforward. When the handwheel is rotated in a clockwise direction, the stem will move downward, pushing the disk towards the seat. Lesser the difference between the seat and plug, the lesser will be the flow and vice versa. In the end, the disk will sit tightly over the seat ring, completely shutting off the fluid flow.
Similarly, the anti-clockwise movement will pull the disk away from the seat, which will open the valve allowing the fluid to flow freely.
For analyzing the cost benefits of a Bellow Seal Valve over a Piston Valve or a conventional Gate / Globe Valve we will consider a 15mm valve operating on STEAM at 150 psi, in three types of valve.
1. Bellow Seal Valve
2. Piston Valve
3. Standard Globe Valve
A) Capital Cost: We can estimate the cost per unit as follows
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S.NoType of ValveCost Per Valve in UD $1Bellow Seal valves75$2Piston Vlave38$3Standerd Globe Valve20$B) Maintainance cost: Assuming that gland leakage will begin within a period of 6 months in a Standard Globe Valve and in a period of 9 months in a Piston Valve, then maintainance costs will be as follows:
Type of ValveCost On SparesCost of labourDowntimeBellow Seal valvesNillNillNonePiston Vlave5$12$2 HoursGlobe Valve1.25$6$1 Hour
Assuming cost of spare piston rings at US$ 5 per pair of rings.
Assuming a down time of 2 hours x labour cost of US$ 6 per hour of maintainance.
Assuming cost of gland packing at US$ 1.25
Assuming a downtime of 1 hour x labour cost of US$ 6 per hour of maintenance.
B) Cost of Downtime & Energy/Media Loss:
We will assume a gland clearance of 1 thou (0.001 inches) for estimation of gland leakage and a cost of US$ 20 per tonne of steam. Steam will escape at a rate of 25 lbs/hr through this clearance (as per steam tables)
Cost Of Loss Of Media/Energy
Bellow Seal valvesNillNillNonePiston VlavePriceless`12$160$ per monthGlobe ValvePriceless6$160$ per monthIf we assume leakage of 11 kgs/hrs/day=264 kgs (11 kgs x 24 hrs) of steam loss per day x 30 days = kgs x steam cost of US$ 20/Ton = US$ 160 per month of energy Loss which can be saved ! (USD 5 per Day Approx.) The cost of down time is priceless and a value as such can not be put on it. It can vary from a few hundred to millions of US$ per day.
D) Total Cost of Ownership (TCO)= 1 + II + III
Type of ValveIIIIIITotalBellow Seal valves75$NillNone75$Piston Vlave38$17$5$/Day (US $ 160/30 Days)55$ + 5$ / DayStandard Globe20$7.25$5$/Day (US $ 160/30 Days)27.25$ + 5$ / Day
So lets estimate that if the leakage is not attended (which is not in most of the plants), the cost of the valve would
be as follows:
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