Controlling
In practice two types of minimum flow control valves have proven successful.

• Two-step control:
  If the flow required falls below the minimum flow, the minimum flow valves opens fully
• Modulationg control:
  If the flow required falls below the minimum flow, the minimum flow valve opens controlled. Flow required plus discharged minimum flow equals the corresponding necessary minimum flow

The advantage of the two-step control is based on the simple interruption-free construction. It can be properly used for operations in small and medium sized plants. The permanent control is more profitable as it saves more energy for large-sized plants and for plants being often started and stopped. For speed-governed feed pumps operation the minimum flow can be adjusted to the rpm of the pump.

You can see the basic construction of a minimum flow plant in the drawing on the left side.

The volumetric delivery is obtained via an orifice plate or a nozzle. The switching equipment signals a contact impulse to the two-step control via a pressure drop variator. The limit contacts which are operated directly or inderectly for opening and closing of the minimum flow valve. Similary the valves are operated by permanent contorl.

For example, the switching equipment is replaced by a permanent controller that can also be locked into the corresponding variable command of the applied rpm-controlled pumps. The minimum flow valve is controlled according to the minimum flow of the required throughput, so that the total necessary minimum flow does not fall below the set limit.

Valve construction and operation
The basic construction of the fittings is shown in the illustrations.

The throttling of the feed water is effected in several stages. The minimum flow valve features a control cone fitted upstream of the throttling device. Through suitable shaping and corresponding stepping of the total pressure on the individual throttles, noise and cavitation ar controlled. Elements prone to severe duty are made of resistant material. Through a practical design of the valve, wear parts are easily replaceable.

Available connections
Fabricated with welded ends (äs shown in the leaflet) with flanges, to meet specs, one side with welded ends, and other side with flanges, one side with welded ends, and other side butt-end or threaded end for Cramp Socket-connection, with minimum flow connection up streams of the valve plate and relief nozzle downstream of the valve plate. Special requirements are considered.

Construction and operation
The Check-Valve casing is fabricated out of solid forged steel piece. The wall thickness is designed for maximum pressure rating and temperature up to 650°C. The Swing check to stop reverse-flow is mounted in the disclever with little axial and radial play and held by screw nuts. The disc-lever can turn freely round the shaft. The movable parts of the disc are fitted separately from the sealing parts. After tightening, the screw is secured and presses the disc firmly against the fourparted thrust ring thus protecting against slipping. The cylindrical pin prevents a horizontal displacement of disc. The casing is provided with a self-sealing cover-plate. This construction guarantees that the valve disc fits firmly even when no pressure is acting, also in a horizontal and vertical piping, where as in vertical Installation it must be made certain that the flow is in the direction from bottom to top. The welding of the valve and heat treatment of the seams may be carried out without removing the internals. The Check Valves may be pickled without harming the internals.

Materials
Materials are choosen to suit the pressure and temperature ranges. For the pressurized parts, preferences is given to the well known types of steel used in boiler and pipeline construction. The spindles are made from corrosion resistant chromium and chromosomal steels, and from high heat resisting chromium steel for high temperatures. Graphite rings are used 1cr packing. The joint faces of the valve are clad in stellite, then precision ground and lapped.
Our gate vavles can be supplied on request as throttle valves or parallel slide valves, the contact pressure of the discs in the case of the parallel gate valves being achieved by means of heat resistant compression springs.