Valves - Gate design

 A gate valve is sometimes not understood at all, although it was thought/desgined for causing the smallest possible pressure drop. Here, I will try to give a summary of its features.

Small gate valve. Notice the shape of the gate and how when fully open you can see through it (meaning that it causes very small pressure drop)

 CONTENTS

 How does a gate valve looks like?

 What to use a gate valve for?

 For which cases a gate valve is not remommended?

 Recommended fluids

 Sizing a gate valve

 Check your knowledge on gate valves

How does a gate valve looks like?

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Well, several images of it can be found all over the internet but as for most general designs it is identified by its obstacle in the form of a gate or disc.

As you can see, in he above image, the design of the valve is not complicated  and can be found in the market in materials such as,

• plastic (PVC or other),
• bronze,
• steel,
• special alloys or other non metallic materials.

Perhaps, the most visible characteristic of a gate valve is its casing which shows, externally, the shape of the gate. This is a very useful way of identifiying a gate valve at first look.

What to use a gate valve for?

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As mentioned early, gate valves were designed, perhaps, with a purpose in mind,

cause the smallest possible pressure drop in a pipe

Why would this be a plausible reason to create a new valve? Some needs in industry wold have driven the creation/design of such a valve. Here is a list of some of these needs,

• a valve, or some other device, allowing isolation of equipments during maintenance, solely, was needed,
• since the valve, or some other device, would only be used during maintenance operations it should be fully open during normal operation,
• since the valve, ir some other device, would only be open/closed it should caused the smallest possible pressure drop in the line,
• a  valve, or some other device, is needed for isolation of parts of the process piping

At this stage, it can be said that flow control was not something needed since other  suitable valves for this purpose already exist.

So, in which situations you would use a gate valve? Those listed above.

For which cases a gate valve is not recommended?

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There are some. A number of situations arise due to too poor understanding of the physics of flow conditions and needs of the application.

• Gate valves are not suitable for flow control. The movable obstacle, gate or disc, may experience loose of tightening due to continued flow stagnation at it. This is not a failure occurring suddenly but it will in time.
• Gate valves are not recommended for quick closing operations. Gate valves are in fact multiturn valves, meaning that several wheel turns are needed to fully open/close it. As gate valves will stop the fluid flow, sudden shuts may cause undesirable hammer effects.

Recommended fluids

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There are several variations to the classical gate valve presented here with modifications such as,

• material of construction,
• ports options,
• gate/disc design,
• actuator mechanism,
• packaging.

However, the nature of the fluid is to be considered as well. For example, not only its temperature, viscosity and pressure but its phase. Gate valves are ideally designed for:

• pure liquids and
• pure gasses.

Unfortunately, in practice working fluids may be present in different ways. For example: as two-phase, which can also be one of the following options,

• liquid-solid
• gas-solid

where different proportions of each phase may occur. 

If the working fluid is pure liquid or a pure gas, no problem related to the hermetic close of the valve would be expected. However, if small particles are suspended in the carrying fluid then clogs may form on the gate/disc path so that hermetic closing is prevented leading to leakage.

Depending on the nature of the suuspended particles,

• shape,
• concentration and
• tendency to agglomerate

a gate valve may not be the best choice for your application. In cases where reasonable doubt exist, you should refer to a reliable manufacturer or supplier.

Sizing a gate valve

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Despite the complexity related to true flow control valves. Gate valves are simple for its application itself: a gate valve is usually installed for equipement isolation during maintenanace or isolation of pipe networks sections, for example. This means that gate valves are not thoght to modify the actual hydraulics already calculated. 

In other words, gate valves usually remain fully open and introduce the smallest possible pressure drop. Therefore, gate valve ports size should be equal to those of the pipe where it is to be installed.

Check your knowledge on gate valves

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Please, follow the link below to find a quizz on this topic and test what you have learned,

What you know about gate valves

This is the end. I hop you find useful the information of this post.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• The pros and cons of bulb thermometers
• The pros and cons of bimetallic thermometers
• Some examples of temperature instruments
• Minor losses  - Formulas
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

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Ildebrando.