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 .
Different Parts of the Globe Valve
- 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.
Construction of Globe valve
Different Parts of the Globe valve ( contd. )
- 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.
Types of Globe valves
Globe valves can be classified in many different ways as follows:
- Depending on whether the valve disc is attached or not attached to the valve spindle: Screw down non- return valve when the two are not secured together and screw lift valve when they are separate.
- Depending on the direction of flow: When the two flanges are in the same line or parallel, they are called in line valve or straight pattern valve and when the flanges are at 90 degrees then they are called angular valves or angular pattern valves. The angular valves are commonly used in boiler feed water and heater drain service and in piping schemes where space is at a premium and the valve can also serve as an elbow. There is another type called the “Y" type of valve where the flanges are parallel, however the valve disc and the valve plate together are at an angle and the outward shape is of the English letter “Y."
- Depending on the material of manufacture: Cast steel, forged steel, bronze, cast iron, nodular cast iron, or steel globe valve.
- Depending on the working pressure 32 bars, 16 bars, 13 bars, 10 bars, and 8 bars, etc.
- Actuator: manually operated, hydraulically operated, or pneumatically operated, etc.
- With screwed ends or with flanged ends.
- Distance between flanges.
Carbon Steel Globe Valve
Maintenance and Operational Care of Globe Valves
The globe valves must be operated in accordance with proper procedures and must be maintained in good leak-free condition by regular maintenance. Some of the points will be discussed now:
- Valve gland packing must be replaced when they have lost their properties. Not changing the gland packings and over tightening the gland nuts to minimize the leakage would make the valve wheel hard, and an attempt to forcibly open or close the valve by wheel spanners and pipes can lead to breaking of the spindle or stem.
- The valves must be regularly greased at the threads and the yoke nut. The grease must be clean and before putting new grease the old grease must be removed. The gland packings, when renewed, must be coated with a liberal amount of grease.
- It is a good practice that when a valve wheel is opened fully, it must be closed slightly (say a quarter turn) as the other operator would know that the valve is open. In case the valve was fully open with the wheel handle tight, the next shift operator might think that the valve is seized in the shut position and forcibly try to open the valve and damage it. But in the case the open valve was shut slightly, the other operator would try to open it and it would open a quarter turn and then become tight. This would indicate to the new operator that the valve was in open position. This practice should be used where ever valve position markers are not present or not working properly.
- Some engineers over tighten a valve when they suspect that it is not holding. This is a wrong practice; a leaking valve should be overhauled and not over tightened. Over tightening may lead to the distortion of the valve plates and breakage of the stem.
- Whenever the pipes are replaced or fabricated, care should be taken that the pipes are made to the correct size and not pushed in position with the help of crow bars and chain blocks. There should be no residual tension on the valve body as that can lead to failure of the valve body subsequently.
- The gaskets that are used in the flanges must be of the correct size. Some junior engineers when cutting the gaskets make a small hole in the center, thus effectively throttling the flow.
Maintenance and Operational Care of Globe valves (contd.)
Overhaul of Globe valves
When a valve is suspected not to hold or it has become tight or seized, it must be dismantled for overhaul. The overhaul procedure has been discussed below:
- Remove the wheel nut and the wheel handle
- Open the bonnet bolts and the top portion of the valve along with the valve seat, stem etc will come out. The bottom portion of the valve will be separate.
- Hold the bonnet in a vice and open the gland bolts. Then loosen the gland flange and the gland bush. The valve stem should then be screwed clockwise and it would come out from below along with the valve disc.
- Remove the set screw of other arrangement that is securing the valve disc to the stem.
- Remove the gland flange, gland nut, and all the gland packings with a gland packing extractor.
Globe valve overhaul:
- Clean, buff, and polish all the parts.
- Check all the threaded parts for damage and if there is any damage on the thread then the threads must be dressed by a needle file.
- Check the stem for straightness. The simplest method of checking is by rolling on a flat surface, but the correct method is to put it on a lathe and center it. Thereafter with the help of a dial gauge put on a magnetic stand at a suitable point, check the distortion.
- Check the stem for scratches, as these can damage the gland packing. Any scratches must be removed with an emery paper or oil stone.
- The valve and the valve seat must be inspected and if found to be in fairly good condition, they must be ground together by using a guide for centering and a suitable grade lapping paste. A 1200 grit lapping paste would be suitable in most conditions.
Correcting Globe valve seat leakage:
- In case the valve seat is extensively damaged, and cannot be repaired by lapping or will take a very long time to lap, the same can be changed if it is replaceable. Some replaceable seats are screwed in, while some are force or shrink fitted. In the case it is shrink filled,the old seat can be taken out by a hammer and chisel. But when it is to be put back, the seat must be kept in the fridge room (meat room -14 deg C approx), or can be cooled by liquid nitrogen or CO2 . After cooling the seat will go in smoothly and after sometime expand and assume position. But if it if built-in type then it must be machined true using a valve seat cutter.
- A valve seat cutter is a special tool to cut the seats of the errant valves. It has different sizes of cutters to suit the different valves. It also has a centering guide to correctly align the tool.
- The seat on the valve disc can be put up on the lathe and machined true. This should be followed by lapping the two parts together.
Assembly of Globe Valve
Clean all parts and blow dust with compressed air.
- Put the valve disc on the stem with the set screw or the arrangement provided.
- Put grease on the threads and screw in the stem through the yoke nut from down below. Care must be taken to put the gland nut and gland flange in correct order in between.
- Put back the wheel handle and the wheel nut and tighten it.
- Now close the valve wheel handle fully, so that the valve stem along with the valve disc extends the maximum down.
- Now put a correct size new gasket on the space provided on the valve body, stick it with grease or Molyslip in case of high temperature applications.
- Now place the valve disc on the valve seat and correctly center it. This is very important in certain valves where there is no guide.
- Now put the nuts and the bolts in place but do not tighten them. There would be some gap between the body and the valve bonnet. From there check again that the valve is properly centered.
- Now start opening the valve. As you do so, the gap between the bonnet and the body would reduce and they would come together.
- Now fully open the valve and tighten all the bonnet bolts in the correct sequence.
- Now put correct size, correctly cut, and well-greased gland packings in the stuffing box place. Care should be taken that the cut parts are together with no gaps and that the packings are staggered.
- Push down the gland nut and the gland flange and put in the gland bolts. The gland bolts must be slightly tightened and the correct tightening must be done in place of installation.
- Now close and open the valve several times and check whether it is free in movement. Check from the opening of the flanges, whether it is sitting properly on the valve seat.
- Paint the valve with the primer and keep it for drying.
Testing of Globe Valve
After the globe valve has been overhauled, it must be tested properly before use. The testing procedure is such that the globe valve is to be filled with water above the valve disc (after the valve is closed), and in first step, the engineer must check whether any water is leaking to the other side. If any leakage is there in the first test, then the valve and the valve seat have to be re-lapped and then re-tested. If the globe valve is now satisfactory and there is no leakage, then the second step of the testing should be done. In it, an appropriate flange has to be put and compressed air has to be put from the lower flange, below the valve seat at a pressure equal or more than the service pressure. If no bubbles are observed on the water at the top, then the valve is considered to be holding and it has been correctly overhauled.
The other method of testing is to put a compressed air connection, with a regulating valve and a pressure gauge to one side of the valve, with good gaskets and tighten it. Thereafter the globe valve should be closed tight, and compressed air at a pressure equal or more is put in and the regulating valve closed. Next the drop in pressure is observed. If there is no pressure drop for five minutes, then the valve is holding and the test is successful.
After the testing has been successful, then the valve should be put in a proper location, with a tag stating that the valve has been tested, the date of testing, and the name of the testing engineer.
Globe Valve Diagram
In this article the classification of the globe valves, their routine maintenance, their overhaul procedures, and the testing procedures have been discussed in brief. It explains to shore and marine engineers the cautions and precautions to be observed during the operation. In our industrial work places and on ships, globe valves have often been taken for granted. Even good planned preventive maintenance plans often overlook them. They are only opened up for investigation when a problem in the flow or pressure, or if any leakage exists. Globe valves are widely used on the ships and are very sturdy, but like all machinery and devices, have to be maintained properly. It is hoped that this guide would create awareness among junior engineers and other marine engineers about the operational precautions and maintenance of globe valves.
- Marine Auxiliary Machinery, 7th Edition by H. D. McGeorge
- Introduction to Marine Engineering, Second Edition, by D. A. Taylor
- Control Valve Handbook , Fourth Edition, Emerson Process Management
Carbon Steel Globe Valve: Wikimedia Commons
Construction of Globe Valve: Tradercity.com
Globe Valve Diagram: Wikimedia Commons
Globe Valve: Wikimedia Commons