Choosing the right valve for large bore sewage systems
Gate valves and knife gate valves are probably the most common styles used in sewage systems. We’ll consider both using the following criteria:
- Valve size.
- Pressure rating.
- Operation (manual or actuated).
- Media – raw sewage or treated sewage.
- Leakage of sewage to atmosphere.
Resilient seated (RSV) gate valves have become the valves of choice where size and pressure rating allow. Their rubber sealing eliminates biggest problem with wedge gate valves – the groove in the bottom of the valve fills up and prevents proper sealing. Sadly RSV gate valves are not currently available in sizes greater than 600mm. This means that large bore systems are restricted to using wedge type gate valves.
- Valve size. This is almost unlimited in wedge gate valves – sizes up to 3000mm (and more) have been supplied over the years.
- Pressure rating. 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.
- Valve size. Although also increasing, an approximate limitation is 1200mm. Sizes larger than this result in very difficult operation and sealing around the blade – and are mostly only practical with bonneted designs.
- 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 often reluctant to declare the actual ratings on large bore valves as they depend on various factors including:
- Percentage sealing required – on most slurry systems 100% sealing is not required. This may be different in sewage systems where often 100% sealing is required.
- Sealing in both directions. A large portion of knife gate valves are uni-directional, and only seal 100% in one direction.
- 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 this has added to the cost.
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.
This article is also available for download here: Comparison of the use of Gate valves vs Knife gate valves on Sewage systems