Schroedahl Type MRK Pump Protection Valves

Product Features

• Automatically modulating bypass function
• Forged housing material, carbon steel or stainless steel
• Special materials possible (e.g. Super Duplex)
• Internal operation (no external energy source required)
• Easy to install
• Low maintenance
• Suitable for all liquids

Applications

• Boiler feed water
• Condensate
• Pumps in fossil fuel plants
• Chemical processes
• Offshore applications (seawater injection)

Technical Data

Function

The pump protection valve for high pressures. The MRK is used as a protection system for centrifugal pumps in water applications.

The MRK valve system comprises a non-return valve and a special control and throttle device for the bypass system.

The general valve functioning is related to the process quantity (flow sensitive). The bypass section itself consists of a primary control unit (multi stage) and a secondary, very special, multi stage pressure regulator. The functioning of the complete bypass is therefore split into two parts. Both parts (primary and secondary) must work together to perform the required pressure drop function. The secondary part is controlled via the outer-connected pressure piping on the bypass section.

The MRK valve protects high pressure centrifugal pumps against overheating and cavitation problems by automatically maintaining a minimum flow when the system flow is at low load condition. At lower process flows, the MRK valve activates the bypass trim parts via a lever system, so that the pump is protected with the correct minimum flow. When the process/system flow starts (increases), the main non-return valve lifts off its seat and starts to operate (modulate) the bypass recirculation flow that returns to the system tank. If the MRK valve detects a sufficiently high system flow the bypass closes automatically (switching point). As soon as the system flow decreases again, the bypass controls automatically.

The automatic recirculation valves usually operate in the load range from 40% to 100% of the rated process flow. The automatic valve handles the typical time limited start-up and shut-down phase by automatically modulating the bypass control operation.

MRK valves at high pressure service may also need an adequate bypass back pressure, e.g. an orifice restriction in bypass line, to prevent cavitation during bypass flow condition.

Full operation range application, which is typical for MRK applications: For high pressure MRK applications with the explicit definition of the full load range from 0% to 100% process flow, it is mandatory before order placement to evaluate design impacts on the valve.

For high load ranges it may be necessary to increase the bypass pressure to avoid cavitation, depending on the existing bypass pressure. This also applies to modulation processes in the bypass line. Therefore, the installation of a special back pressure valve BPV is recommended for the full operation range application to ensure that the bypass pressure level is always at a suitable level.