KRACHT KF D15/D25 Gear Pump with Pressure Relief Valve

KRACHT KF D15/D25 gear pump with attached pressure relief D-valve

The KRACHT KF D15/D25 designation describes a KF series external helical-gear pump fitted with the directly controlled pressure relief valve known as the D-valve. The valve is bolted to the pump housing and does one job: it opens when pressure rises above its setting, so that a short-lived pressure spike does not reach the gear set, the shaft seal or the pipework. Seven Star LLC supplies KRACHT KF pumps and valve options in Oman and the GCC as an independent supplier in Muscat, and this page explains what the KF D15/D25 option is, how its code is read and what to send us for a quotation.

In the KRACHT type key the valve sits in position 7. The letter D means the directly controlled relief valve and the number is the top of the adjustment band in bar: D15 is adjustable from 0 to 15 bar and D25 from 15 to 25 bar. A pump written as KF 40 R F 1 / D15 therefore means a 40 cm³/rev pump, right-hand rotation, DIN flange without outboard bearing, sealing type 1, with a D-valve in the low band. This page covers the D15/D25 pair; the T-valve and universal valve variants of the same pump family have their own pages because their purpose and limits differ.

Technical specifications of the KF D15/D25 pump and valve

How the D-valve protects a gear pump

A gear pump is a positive-displacement machine, so it keeps delivering volume even when the line downstream is blocked. Pressure then climbs until something gives way: a seal, a hose, a coupling or the relief valve. The D-valve is the planned weak point. It sits between the discharge and suction passages in the valve housing, and its adjustable setting decides the opening pressure. When discharge pressure exceeds that setting, the valve opens and the overflow is returned to the suction side of the same pump.

That internal return is the key limit of the KF D15/D25 option. The oil recirculates inside the pump, it does not go back to the tank, so it warms up with every pass. KRACHT states that the valve is designed for a brief overflow only and that it may be used as a safety valve for short-term operation. If a part of the flow has to be drained for a long period, a separate relief or pressure control valve with a return line to the reservoir is required. For such duty KRACHT offers SPV, DV B, DV R and T type valves with an external outflow.

Where the KF D15/D25 fits in the system and related options

The D-valve is one of four valve choices for the KF family. The KF T15/T25 option adds a separate tank connection for continuous bypass of sizes 32 to 80, and the KF U/U2 universal valve keeps the same port as suction or pressure when the shaft reverses. A KF D15/D25 pump differs from the plain version, and a KF pump with no valve is the plain version, and only that version can be ordered in direction B. The D-valve can be combined with any of the 26 nominal sizes, which makes it the most widely used of the three attached valves.

Typical pairings are a KF D15 on a lubrication or fuel-transfer set where the line pressure is low and a KF D25 on a hydraulic power unit or a gear-oil circuit running closer to the 25 bar limit. The setting band is chosen so that the valve opens a little above normal working pressure and well below the 40 bar certification test pressure of the smaller pump sizes. An in-line safety valve is still advisable on any installation where a blocked line is realistic. When you compare a KF D15/D25 with the other KF valve options, remember that the KF D15/D25 is the simplest and the only valve option available on the full size range.

Selection and ordering checklist for KF D15/D25

• Nominal size in cm³/rev and the required flow at the real motor speed, 950 or 1450 rpm and whether a variable-frequency drive is used.
• Working pressure and the highest pressure expected in upset conditions, to decide between D15 (0 to 15 bar) and D25 (15 to 25 bar).
• Direction of rotation R or L as seen on the shaft; a wrong rotation destroys the shaft seal.
• Mounting: DIN flange or mounting angle, with or without outboard bearing if a belt or radial load acts on the shaft.
• Sealing type number from the sealing table, for example 1 for NBR lip seal or 2 for FKM, or a mechanical seal where leakage is not acceptable.
• Housing material, grey cast iron as standard or GJS for higher shock loading.
• Fluid, viscosity at minimum and maximum temperature, and any solids or water content.
• Connection type: Whitworth thread or SAE flange for sizes 2.5 to 25, SAE flange for sizes 32 and above.

Installation, commissioning and maintenance of the KF D15/D25

Fill the pump housing and the suction line with the process fluid before the first start. Dry running quickly damages the plain bearing bushes. Check the direction of rotation with the coupling uncoupled, because the shaft seal is destroyed within seconds if the pump turns the wrong way. The relative suction pressure at the pump inlet should not fall below –0.4 bar, and only briefly to –0.6 bar during a cold start.

Set the D-valve while running, with a pressure gauge on the discharge. Turn the adjusting screw until the valve just starts to open at the desired pressure and lock it. Never rely on the D-valve to hold a continuous bypass; if the housing becomes hot to the touch while the valve is open, the circuit needs an external relief valve. In service, watch for rising housing temperature, increased noise (which points to cavitation or air in the oil), leakage at the shaft seal and a gradual drop in delivered flow, which signals internal wear. A filter finer than 60 µm protects the gear set and the valve seat. Keep a spare shaft seal kit on the shelf for critical units, and record the KF D15/D25 type code and valve setting on the maintenance sheet so that a replacement is configured identically.

Typical applications of KF D15/D25 pumps in Oman and the GCC

Gear pumps with a D-valve are common in lubrication and gear-oil circuits for compressors, turbines and large gearboxes, in diesel and fuel-oil transfer sets at power stations, and in hydraulic units for valve actuators and cranes. In Oman and the Gulf the main design concerns are high ambient temperature, dust and long idle periods. Standard painting is class C2m, so for coastal desalination or petrochemical sites a C4 or C5 corrosion class should be specified at ordering, along with FKM or PTFE seals when the oil runs hot. Where the pump sits in a classified area, the ATEX version for sizes 2.5 to 200 is the relevant choice.

KF valve option family

Information to send with an RFQ for KF D15/D25

• Full KRACHT type code from the nameplate, or the nominal size, rotation, mounting and sealing choice.
• Fluid, viscosity range and operating temperature.
• Working pressure and valve band, D15 or D25.
• Drive: electric motor power and speed, or other prime mover, and the coupling or belt type.
• Connection type and size, ATEX requirement and surface protection class.
• Quantity, delivery site and whether spare seals are required.

Documents and quotation

The manufacturer data sheet for the KF 2.5 to 630 series, covering the version without valve, with D-valve and with T-valve, is attached on this page. It contains the type key, working characteristics, sealing and special-number tables and the dimensions of the pumps with D-valve.

Send the KF D15/D25 type code or the duty data and we will confirm availability, lead time and a quotation for your KF D15/D25 requirement in Oman and the GCC.