Knife gate valves vs. gate valves: which is the right choice for your application?

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A knife gate valve and a gate valve have a lot of similarities that are apparent at first glance, so they are often discussed together. However, there are a lot of differences between these two valves, which makes them suitable for different applications. If you&#;re wondering which valve is the best for you, this knife gate valve vs gate valve comparison from BM Engineering will give you all the information you need to make the right decision.

 

What are knife valves?

A knife gate valve is a type of shut-off valve. These are used to start and stop the flow of media through a pipeline. Knife gate valves are predominantly seen in applications where there are solids in the media. The &#;knife&#; aspect refers to the blade or gate of the valve which cuts through the media.

 

What are gate valves?

A gate valve is typically used for shut-off operations, like a knife valve, as it only uses on and off settings. These also securely stop the flow of media when required. Due to their simple design, gate valves are often applicable for processes that need a straight flow of fluids with little to no restrictions.

 

Key knife gate valve vs gate valve differences

Despite their relatively similar function as shut-off valves, these two valves are used for different applications. Some of the applications for a knife gate valve include:

 

• Non-flammable viscous fluids

• Wastewater

• Heavy oils

• Slurries

 

Gate valve applications include:

• Clean Water

• Any clean, non-corrosive media which is free from solids

• Gate valves are not suitable for steam

Although somewhat similar in function, a knife gate valve vs gate valve comparison demonstrates several differences that distinctly separate them.

 

Due to the sharpened gatethat features on a knife gate valve, they are better for use in applications that require a thick substance to be cut through to stop flow than a gate valve would be. In applications involving more non-viscous media that doesn&#;t need to be cut through, a gate valve would be more applicable.

 

Additionally, knife gate valves are slimmer in profile to gate valves, so a similarly sized knife gate valve will weigh less than a conventional gate valve.

 

Another significant difference when it comes to the knife gate valve vs gate valve debate is around the packing gland area. A gate valve has a v-ring packing set that seals the shaft attached to its gate, while a knife valve has a packing gland that seals around the gate.

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A knife gate valve is usually uni-directional (though bi-directional is an option on some models), and comes in either a lugged or wafer body with no flanges. The seats that feature on a knife gate valve are available in metal, resilient types and more.

 

Due to all these differences, knife gate valves and gate valves are used in distinct applications, despite there being some occasional overlap. Knife gate valves are usually found in environments such as chemical, cement and wastewater treatment plants that work with thicker media. Gate valves are typically found in general fluid processes, drinking water pump stations and textiles where they are better suited to thinner fluids.

 

Knife valves and gate valves at BM Engineering

At BM Engineering, we stock a vast range of valves suitable for an equally vast selection of industries. Now that you know all about the differences through this knife valve vs gate valve comparison, you can select which one is most ideal for your budget and application. We stock knife valves, gate valves, butterfly gate valves and plenty more on our website.

If you need further assistance regarding the best valve for your process or anything else valve related, don&#;t hesitate to contact us today at 762 , or us at .

Comparison of the use of Gate valves vs Knife gate valves on Large Bore Sewage systems

These are traditionally probably the most common valve styles used in Sewage systems.  With the evolution of valves in general, choices have changed over the years and we try to provide the modern alternatives in this article. 

Criteria to investigate

• Valve size
• Pressure rating
• Operation (manual or actuated)
• Media &#; raw sewage or treated sewage.
• Leakage of sewage to atmosphere

Gate valves

The development in gate valves have dictated that Resilient Seated (RSV) Gate valves have become the valves of choice where size and pressure rating allow. RSV gate valves have the advantage of rubber sealing which eliminates the &#;Achilles Tendon&#; of Wedge gate valves ie the &#;groove&#; in the bottom of a Wedge gate valve and which can fill up with particles which can prevent proper closure of the valve. Although sizes of RSV gate valves are continuously increasing, the current largest size available is mm. For the purposes of this article which deals with Large Bore Sewage systems, we will restrict the investigation to Wedge Gate valves.

Wedge gate valves have been used for many years in Sewage systems with good success and with the following points to consider:

• Valve size. This is almost unlimited in Wedge gate valves &#; sizes up to mm (and more) have been supplied over the years.
• Pressure ratings. Up to 100bar readily available and with production in South Africa.
• Operation. Handwheel and/or Gearbox for Manual operation. Electric actuator for remote or local automatic. Because of the forces required to operate large bore gate valves it is a definite benefit to operate them Electrically. 
• Media. If there are a lot of large particles in the media and which cannot get dissolved, Wedge gate valves have the problem of trapment in the bottom groove of the valve. Generally in Sewage systems the large particles have been eliminated/mulched and the Anaerobic action will dissolve smaller particles. This is one of the main reasons why the weakness of the Wedge Gate valve&#;s bottom groove has not been a major issue with the correct operation of these valves in Sewage systems. If any rags or soft material does get trapped in the bottom groove, the relatively large forces to close large bore Wedge Gate valves (gearbox or Electric actuator), will tend to shred these materials and flush them away.
• Leakage of Sewage to atmosphere. This is obviously to be avoided wherever possible. Wedge Gate valves have only the Stem seal where sewage can leak out. Good designs of stem seal have eliminated this concern as well. 

Knife Gate Valves

The Knife Gate valve has been developed to cope with media which carry particles, slurries and sewage. The reasons for their success in these applications are related to Price (generally made in wafer style with less material), Compactness (short face to face), abrasion resistance due to Stainless Steel blade material and various linings on the body, generally no bottom groove where particles can be trapped.

Because the blade is generally unsupported over its travel, pressure ratings and sizes are limited. One of the major problems with most Knife gate valve designs is the sealing around the blade. Different designs of seal have various benefits, but they all suffer from high maintenance and the fact that the seal acts like a handbrake and makes the Knife gate valve generally difficult to operate. These inherent design problems have restricted the knife gate&#;s size and pressure ratings. 

Various design developments (such as Bonneted styles) have improved the capabilities of Knife gates by eliminating the Blade seal. The following criteria provides a comparison with Wedge Gate valves:

1. Valve Size. Although also increasing, an approximate limitation is mm. Sizes larger than this result in very difficult operation and sealing around the blade &#; and mostly only practical with Bonneted designs.
2. Pressure Rating. Very limited and decreasing as the valve size increases. Generally sizes from 500mm upwards are only rated for 2 to 3 bar. Manufacturers are reluctant to declare the actual ratings on large bore valves as they depend on various factors such as:
3. Percentage sealing required &#; on most slurries systems 100% sealing is not required. This may be different in Sewage systems where often 100% sealing is required.
4. Sealing in both directions. A large portion of Knife gate valves are Uni-Directional, and only seal 100% in one direction. 
5. Capability of cycling under full pressure differential. Pressure ratings of Knife gates generally are governed by the capability of the blade to handle the differential pressure of the system. A lot of manufacturers will state that the valve can handle such a differential pressure only in static conditions but that one cannot cycle the valve under these conditions.

The above factors all indicate the limitation of pressure ratings of large bore Knife Gate valves and care has to be taken in getting guarantees from the manufacturer that the valve can cope with stated conditions.

• Operation. Large bore Knife gate valves are difficult to operate. Manual operation will often result in the valve not being closed properly, resulting in leakage and damage to the blade and sealing area. It is always recommended to operate knife gate valves by means of an actuator which on large bore valves add only a small percentage cost.
• Media. Although Knife gate valves have been designed for media with particles in, care should be taken that the design is such as to eliminate trapment of particles on the bottom of the knife gate valve. 
• Leakage of Sewage to atmosphere. This is a traditional problem with Knife gate valves. Bonneted style Knife gates have gone a long way to eliminate this problem, but has added to the cost.

Summary

All the above factors should be carefully considered in selection of the right valve for Large Bore Sewage applications and process conditions should be carefully considered. It is the author&#;s opinion that overall, Wedge Gate valves are a better selection for large bore Sewage applications.

Peter Telle  (Bsc Mech Eng)

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