Selecting the correct valve for control applications normally poses many challenges to control engineers. In applications where fluid media with a significant solid content is difficult to control with conventional valve designs - operational problems, maintenance and service life require thorough consideration.
Helping to ensure that technologically advanced, quality products are available to meet the demands of control engineers is the Control Division of DLM. "We identified that the southern African market was in need of a cost effective knife gate valve that would provide years of trouble-free and maintenance-free operation even under the most severe service applications", says DLM's Control Division director Marc von Ruben. "Worldwide research into available products lead us to Vaas Industries, an organisation with capability to design, manufacture and test one of the widest ranges of knife gate valves internationally.
"Vaas knife gate valves were developed to cater to specific services where conventional valve designs proved to be inadequate. For instance, fluid media with significant solid content is generally difficult to handle with conventional valve designs such as gate, globe, ball, plug or butterfly valves. We therefore decided to enhance our extensive product range by the addition of this excellent valve."
Vaas knife gate valves are designed to ensure minimum contact between moving parts of the valve. The gate is held between strong layers of gland packing and does not contact the body until it actually seats. The seating is accomplished with a non-sliding motion of the gate towards the body (seat) aided by jams located in the body. Avoiding sliding contact between gate and body permits this valve to handle abrasive particles present in the media more effectively than conventional designs.
The knife gate valve is a unidirectional low-pressure valve ranging in size from 50 mm to 1200 mm with pressure ratings up to 1 MPa (10 bar). Modifications that offer bi-directional shut-off are also available.
The bevelled bottom edge of the gate (from which it derives the 'knife' in its name) allows positive closure of the gate even when solid particles have settled at the bottom of the body. Deflection cones are used to avoid direct impingement of abrasive media on the seat surfaces, which themselves are hardened by either stelliting the seat/gate or nitride coating the entire body and gate.
The purge connection feature allows suitable fluids at pressures higher than line pressure to clean seating surfaces (body purges) and packing area (chest purges), minimising abrasive wear and tear during valve opening and closure.
Knife gate valves are normally round-port designs, however, square and rectangular port variants are also available which are typically used in hopper isolation applications. This is the only valve design that tolerates differential thermal expansion of its parts without affecting its functionality, thus making the Vaas knife valve an ideal choice for high temperature media.
Another variation is the v-orifice design that offers a compromise between controllability and capability to handle media containing solids such as sludges or slurries. Vaas knife gate valve design variations include two-lug, full lug wafer and flanged designs; metal-to-metal and resilient seat options; v-orifice, square and rectangular port designs; options such as purges and deflection cone; choice of manual, pneumatic, electric and hydraulic actuators. With the correct selection and installation process, the Vaas knife gate valves will provide long service life and maintenance-free operation even in the most difficult service situations.
For more information contact Marc von Ruben, DLM, 011 457 0500, [email protected], www.dlm.co.za
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