Control Valves

Control valves are the final element of a control loop: they throttle flow continuously in response to a signal from the DCS or PLC. Seven Star LLC supplies control valves and their spare parts in Oman, with 115 listed items led by Masoneilan and followed by Fisher, Flowserve and Samson.

The category also includes Moog servo valves for hydraulic control and a sub-category of pressure independent control valves for chilled water (CHW) systems.

Globe, rotary and severe service control valves

Sliding-stem globe valves are the general-purpose choice. The Masoneilan 21000 Series is a top-guided globe valve, the 41005 Series is cage-guided, and the 80000 Series three-way valves mix or divert flow. Rotary designs give more capacity for a given line size: the Masoneilan 35002 Camflex and the Fisher CV500 are eccentric plug valves, and the 39003 and 33000 Series are high performance and triple offset butterfly valves. For high pressure drops, multi-stage trims such as the 77000 labyrinth design reduce pressure in steps to limit cavitation, noise and erosion. A large share of the listing is seal kits, packing kits and trim parts for these series, so a worn valve can be overhauled instead of replaced.

HVAC control valves and servo valves

Pressure Independent Control Valves from Belimo, Danfoss, Frese and Siemens hold a set flow through fan coil and air handling units regardless of pressure changes elsewhere in the chilled water network. Moog pilot-operated servo valves belong to hydraulic motion control, where spool position follows an electrical command. pFlow flow control valves complete the list; Seven Star is the agency for pFlow in Oman and the GCC.

How a control valve is sized and specified

Sizing starts from the process data: fluid, flow rates at minimum, normal and maximum, inlet and outlet pressures, and temperature. From these the required flow coefficient (Cv) is calculated to IEC 60534, and a body size and trim are chosen so the valve works in the middle of its travel at normal flow. The inherent characteristic, usually equal percentage or linear, is matched to the process. Seat leakage class is defined by ANSI/FCI 70-2.

What to include in a control valve inquiry

Send the valve datasheet if one exists. Otherwise give size, pressure rating and end connections, process conditions or required Cv, trim type and characteristic, body and trim materials, shut-off class, actuator type with air supply and fail position, and the positioner signal. For spare parts the valve serial number is the most reliable reference.