Lee Pressure Relief Valves
How to select the right Relief Valve for your application
For Pressure relief and sequencing:
  • High Capacity Miniature Inserts
  • Available in Cracking Pressures from 20 - 5400 psid
  • Models Available for Low or High Flow Rates
For Thermal relief:

  • Optimized for Relieving Small Flows
  • Low Leakage
  • No Elastomers
  • Available for 3000, 4000 and 5000 psi Systems

187, 250 and 281 PRI
The 187, 250 and 281 PRI Valves are specifically designed to be the smallest valves available for safety relief applications. The valves are constructed entirely of stainless steel and are available screened and unscreened for design flexibility. Built-in damping assures quiet operation. The 281 PRI is also available in a zero leak model.

The 187 PRI is the smallest relief valve, only three sixteenths of an inch in diameter and a little over a half inch long, covering cracking pressures from 20 to 100 psid. The 250 PRI and the 281 PRI covers cracking pressures from 80 to 3600 psid. Cracking pressures for the 281 Zero Leak PRI are from 800 to 4700 psid.

375 and 500 Momentum PRI
The 375 and 500 Pressure Relief Valves use the patented “momentum free” design concept. The unique design compensates for momentum forces that upset the normal force balance of a direct acting valve. These forces are produced by local pressure gradients caused by changes in liquid velocity. (See Momentum Forces). The result is a valve that is both linear and stable throughout the operating range and has low hysteresis, yet is still miniature. The 375 PRI has full open flow capacity of 100 Lohms and the 500 PRI has a 50 Lohm capacity. Both are available in standard cracking pressures from 50 to 500 psid.

281 and 375 High Pressure PRI
The High Pressure PRI is designed for pressure relief service where operating pressures and pressure spikes are too severe for conventional safety relief valves. These conditions, when encountered by conventional valves can lead to instability, erosion and other damaging phenomena

The 281 High Pressure PRI is designed to overcome such problems by utilizing a special poppet design in conjunction with a rugged seat design.

The 375 High Pressure PRI is a relief valve designed for optimal performance in applications with high backpressure. This relief valve has shown stable relief flow performance in applications with backpressures up to 850 psig. It features a durable tungsten carbide ball with the remaining components constructed entirely of stainless steel. Available in forward and reverse relief flow directions, the 375 High Pressure PRI offers a range of cracking pressures for 3000, 4000 and 5000 psi systems with system peak pressures up to 6750 psi.

281 NACE Compliant PRI
The 281 NACE Compliant PRI is made with materials specifically chosen for optimal performance in harsh duty applications, such as those in the oil tool industry. This high speed, direct acting valve is constructed entirely of corrosion-resistant materials that are compliant with NACE specification MR0175/ISO 15156. A screened and unscreened model is available for system pressures up to 3000 psi. A high pressure screened model is also available for system pressures up to 5000 psi.

Hi-Q Pressure Relief Valve
The Hi-Q Pressure Relief Valve, sometimes referred to as an LRU (Line Removeable Unit), is designed for applications that require both a high relief pressure and a high flow rate in a compact package. The valve is capable of relief pressures up to 5400 psid and flow rates over 19 GPM. The valve utilizes a side inlet flow path to help minimize the installation boss size and weight.

375 PRI/Chek
The 375 PRI/Chek Valve combines the function of a pressure relief valve in parallel with a check valve into one easy to install insert. The valve is available in a range of relief flow cracking pressures for system pressures up to 5000 psi.

187 TRI and 250 High Pressure TRI
The Lee TRI has been designed to relieve fluid pressure caused by thermal expansion. When a sealed container is completely filled with fluid, thermal expansion can cause container rupture or seal failure. The Lee TRI eliminates this hazard.

Because many systems need to relieve only a few drops of fluid to return to their design pressure, the Lee 187 TRI has been optimized for relieving small flows. A high Lohm rate restrictor is used in series with a relief valve to limit the flow rate. This restricts the flow rate sufficiently to ensure shut off above system pressure. An advanced technology ceramic ball and seat material provides the toughness and dimensional stability needed for a tight shutoff. The incorporation of an integral 15µm CRES filter upstream of the valve seat solves the low flow contamination problem. The Lee TRI is not recommended for general pressure relief service. Excess total flow through this valve may cause clogging of the integral filter.

Lee also offers a 250 High Pressure TRI that is a blend of the 281 High Pressure PRI and 187 TRI technologies. This valve is intended for thermal relief applications where the thermal relief valve will experience severe pressure spikes or pressure change rates in excess of 25,000 psi per second. The valve's tight hysteresis and rugged design make it the preferred selection for these types of applications.

SELECTING A RELIEF VALVE
Lee relief valves come in a wide range of styles. Each style has been developed to satisfy specific relief needs commonly found in hydraulic systems. These may vary widely from the intermittently exercised safety “pop-off” application, to the continuously running pressurizing valve or the special requirements relieving a few droplets of oil from a trapped volume being heated.

Since these applications are very different, it is important that the correct valve is selected to ensure proper functioning of the system. This introduction will help you to make that selection.

SELECTION CRITERIA
Preliminary selection criteria are the Lohm rate at the “flow-point” (calculated from the flow and differential pressure using the Lohm law) and the cracking pressure. Use the chart below to find the valve types with a suitable range of cracking pressures. Then choose a valve type with a Lohm rate lower than the calculated number.


The final selection involves matching the valve’s operating characteristics with the desired function in the hydraulic system. The principal characteristics of each valve type are tabulated below.

VALVE TYPE
TYPICAL APPLICATIONS
PRES. BAND*
%
LOHMS AT MIN. VALVE LIFT**
SUITABLE
FOR
METERING PARTLY OPEN
SUITABLE FOR THERMAL PRESSURE RELIEF
187 PRI1) General L.P. relief
2) Pressurizing valve
3) As a check valve
40
1500
Yes
No
250 PRI1) General low-flow relief
25
Not Limited
Yes
No
281 PRI1) Safety "pop-off" relief
2) Cross port load alleviation
40
1500
No
No
375 Momentum PRI1) Filter by-pass relief
2) Lubrication systems
3) Fuel systems
35
500
Yes
No
500 Momentum PRI1) Filter by-pass relief
2) Lubrication systems
3) Fuel systems
35
350
Yes
No
281 High Pressure PRI1) Combination thermal pressure relief and low flow relief
2) General low-flow relief
25
Not Limited
No
Yes
375 High Pressure PRI1) General High Pressure relief
2) Cross port load alleviation
28
1500
Yes
No
Hi-Q PRI1) General High Pressure relief
2) High flow rate
varies
600
Yes
No
187 TRI 1) Thermal pressure relief only
25
Not Limited
Yes
Yes
250 High Pressure TRI1) High pressure thermal relief only
25
Not Limited
Yes
Yes
* Pressure Band defined as: [(pressure at flow point – pressure at shut off) / cracking pressure] x 100
** Lohms at minimum valve lift is derived from the minimum valve opening necessary to avoid silting.


ADDITIONAL CONSIDERATIONS FOR
THERMAL PRESSURE RELIEF
Given a system volume and maximum temperature rise rate, you can find the relief flow rate caused by thermal expansion of the fluid. The Nomogram is for Skydrol 500, but may be used with other fluids having approximately the same coefficient of cubical thermal expansion (MIL-H-5606). Other fluid flows must be calculated using the formula and coefficients found on pages M24 – M25 of the Lee Technical Hydraulic Handbook.

Compare the flow rate calculated with the flow at the Flow Point for a TRI valve with a suitable cracking pressure (see Table on page F28 or F30 of the Lee Technical Hydraulic Handbook). If the calculated flow rate is less than the one shown in the table, the Lee TRI will limit thermally induced pressure in your system to no more than the Flow Point pressure shown.
LEE TRI FLOW NOMOGRAM
FOR SKYDROL 500 OR MIL-H-5606 AT 100°F



Datasheets
Hide details for [<br><br><img src="AWM0521L.GIF">]

Hide details for [<br><big>] 187, 250 and 281 PRI [</big><table><td>] The 187, 250 and 281 Pressure Relief Valves are specific
187, 250 and 281 PRI
The 187, 250 and 281 Pressure Relief Valves are specifically designed to be the smallest valves available for safety relief applications. The valves are constructed entirely of stainless steel and are available screened and unscreened for design flexibility. Built-in damping assures quiet operation.
Ø.187" PRI - Unscreened
Ø.187" PRI - Screened
Ø .281" PRI - Unscreened
Ø .281" PRI - Screened
Ø.281" Zero Leak PRI
Ø.250" PRI
Hide details for [<br><big>] 375 and 500 Momentum PRI [</big><table><td>] The 375 and 500 Pressure Relief Valves use the paten
375 and 500 Momentum PRI
The 375 and 500 Pressure Relief Valves use the patented 'momentum free' design concept. The unique design compensates for momentum forces that upset the normal force balance of a direct acting valve. These forces are produced by local pressure gradients caused by changes in liquid velocity. (See 'Momentum Forces'). The result is a valve that is both linear and stable throughout the operating range and has low hysteresis, yet is still miniature. The 375 PRI has full open flow capacity of 100 Lohms and the 500 PRI has a 50 Lohm capacity. Both are available in standard cracking pressures from 50 to 500 psid.
Ø .375" Momentum PRI
Ø .500" Momentum PRI
Hide details for [<br><big>] 281 and 375 High Pressure PRI [</big><table><td>] The 281 and 375 High Pressure Relief Valves are
281 and 375 High Pressure PRI
The 281 and 375 High Pressure Relief Valves are designed for pressure relief service where operating pressures and pressure spikes are too severe for conventional safety relief valves. These conditions, when encountered by conventional valves can lead to instability, erosion and other damaging phenomenon.
Ø .281" High Pressure PRI
Ø .375" High Pressure PRI
Hide details for [<br><big>] 281 NACE Compliant PRI [</big><table><td>] The 281 NACE Compliant Pressure Relief Valves are desi
281 NACE Compliant PRI
The 281 NACE Compliant Pressure Relief Valves are designed for optimal performance in harsh duty applications, such as those in the oil tool industry. These direct acting valves are constructed entirely of corrosion-resistant materials that are compliant with NACE specification MR0175/ISO 15156.
Ø .281" NACE Compliant PRI -- (up to 3000 psi system pressure)
Ø .281" NACE Compliant PRI, Screened -- (up to 3000 psi system pressure)
Ø .281" NACE Compliant PRI, Screened, High Pressure -- (up to 5000 psi system pressure)
Hide details for [<br><big>] Hi-Q Pressure Relief Valve [</big><table><td>] The Hi-Q Pressure Relief Valves, sometimes referre
Hi-Q Pressure Relief Valve
The Hi-Q Pressure Relief Valves, sometimes referred to as LRUs (Line Removeable Units), are designed for applications that require both a high relief pressure and a high flow rate in a compact package. The valves are capable of relief pressures up to 5400 psid and flow rates over 19 GPM.
Hi-Q Pressure Relief Valve-- (up to 5000 psi system pressure)
Hide details for [<br><big>] PRI/CHEK [</big><table><td>] The PRI/CHEK Valves combine the function of a pressure relief valve
PRI/CHEK
The PRI/CHEK Valves combine the function of a pressure relief valve in parallel with a check valve into one easy to install insert.
375 PRI/CHEK Valve-- (up to 5000 psi system pressure)
Hide details for [<br><br><img src="AWM0522L.GIF">]

Hide details for [<br><big>] Thermal Relief Valve [</big><table><td>] The Lee TRI has been designed to relieve fluid pressure
Thermal Relief Valve
The Lee TRI has been designed to relieve fluid pressure caused by thermal expansion. When a sealed container is completely filled with fluid, thermal expansion can cause container rupture or seal failure. The Lee TRI eliminates this hazard.
Ø .187" TRI
Ø .250" High Pressure TRI

For more information, please send us your application information via our ASK LEE service. Or contact our application engineers in the USA at 1-800-LEE-PLUG or our representatives around the world.
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