Uni-Directional or Bi-Directional Knife Gate Valves - What's The Difference?

What's the difference between a uni-directional & bi-directional knife gate valve and which one do I need?

A uni-directional knife gate valve is an economic general-purpose valve that’s easy to operate both manually and actuated. Uni-directional knife gate valves, seal on just one side. They also tend to be prone to leaking when there is a possibility of debris build up, this is because of the cutting action of the blade. Suitable for water, paper and pulp slurries, both mining, chemical and some sticky slurries, fly ash and sugar mills amongst many others. The uni-directional knife gate valve can also be used in harsh aggressive environments, high temperatures

Bi-directional knife gate valves will seat on either side of the gate and operate with back pressures, providing adequate water tightness and has a tendency of avoiding any build-up of solids that could prevent the valve from closing. The Bi-directional valve is generally more expensive, and you can expect to pay up to 30% more. Bi-directional valves tend to have a lower torque/thrust over the Uni-directional valves and they have the added advantage of being able to be fitted on “end of line”.

Selecting between Knife gate valves is tricky and there are trade-off’s so unless you are sure it would be advantageous to ask the question for peace of mind.

Bray Knife Gate Valves

Knife Gate Valves

The purpose of a knife gate valve is to cut through solid or semi solid flows of heavy liquids utilising a blade unlike a slide gate valve which is a valve that uses a slide plate in order to control the flow of dry bulk material.

Knife gate valves utilise a blade with a sharp or bevelled edge. This blade is designed to seat into a metal or soft elastomer seat. The blade cuts through the liquid which glides through slurry and thick liquids quickly and easily.

These knife gate valves were originally designed for the paper and pulp industry, where they found that other valves were prone to clogging. The benefits of these knife gate valves were obvious and in addition could be produced cheaply which suited the paper industry.

Being easy to actuate and to maintain, the benefits of this valve proved quickly to adapt to other markets and as a result became widely used into other industries such as mining, waste water, power plant and food & beverage.

Today knife gate valves can be found everywhere, in processing, bio-fuel and industrial plants. They are available with various operators, supplied in different materials and sizes.

These valves do have limitations though and are restricted to low pressures and not recommended with gases or powder. When dry solid powder passes through a knife gate valve, it tends to pack into the soft seals around the gate and the seat over a time will wear, eventually the gate will not close and start to leak dry dust. In this situation, you will need to replace the knife gate’s seal once it begins to leak and depending where the valve is fitted it may be necessary to remove the entire valve out of the pipe line which in some cases can be difficult because of the size of the valve. So be careful when selecting a knife gate valve when what you are really looking for is a slide gate valve!

Application Selector

Series 740 Bidirectional Knife Gate Valves - Slurry

One-piece cast body with flexible wire reinforced elastomer seat for bidirectional zero leakage shutoff in a wide range of industrial applications.

Engineered for on/off service and isolation of clean, dirty, corrosive or viscous media in pulp & paper, chemical, mining, power and wastewater applications. e.g. Raw & Treated Water, Liquor Service, Paper Stock.

Series 940 Unidirectional Knife Gate Valves – General Purpose

One-piece cast body designed for rugged performance in applications ranging from general purpose to severe media handling.

Engineered for general purpose on/off service and isolation of clean, dirty, corrosive, abrasive, viscous and high temperature media in power, mining, pulp & paper, cement, carbon black and chemical applications.