Pilot Op Relief

8/9/2019 Pilot Op Relief

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ME 4232

Fluid Power Control Laboratory

Pilot Operated Pressure Relief Model

Fall 2007 December 20th

Adam Kalthoff

Andy Marass

Shelley Fabry

Cameron Muelling


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made for the valve and because of that, this is where the diversion of the full flow volume is

accomplished by the balance piston. The balance piston is named this because this is where the

hydraulic balance takes place within the valve. The pressure of the inlet acting under the piston is also

felt on top of the piston by the means of an orifice that is drilled in the balancing piston. This allows the

piston to be held on its own seat by the means of light spring. When the pressure in the valve reaches

the setting that lifts the poppet in the pilot stage off the seat there is a pressure decrease in the upper

chamber of the second stage piston which results in an unbalance in the hydraulic forces. When the

hydraulic imbalance overcomes the mechanical force of the spring the poppet is unseated. After the

pressure difference between the upper and lower chambers of the valve is sufficient enough, the

balance piston will completely unseats itself and allow full-flow directly to the reservoir of the system.

This will continue until the pressure through the valve reduces the system pressure to a pressure less

than the pressure setting of the valve.

Valve Choice

The current pilot operated relief valve used in the Fluid Power Lab was chosen to be the valve used for

the model. The valve used is lab is aRPEC-KDNfromSun Hydraulics. A diagram of the valve is shown in

figure X.
http://www.sunhydraulics.com/cmsnet/GenPDF/RPEC.pdfhttp://www.sunhydraulics.com/cmsnet/GenPDF/RPEC.pdfhttp://www.sunhydraulics.com/cmsnet/GenPDF/RPEC.pdfhttp://www.sunhydraulics.com/cmsnet/sun_homepage.aspx?Lang_Id=1http://www.sunhydraulics.com/cmsnet/sun_homepage.aspx?Lang_Id=1http://www.sunhydraulics.com/cmsnet/sun_homepage.aspx?Lang_Id=1http://www.sunhydraulics.com/cmsnet/sun_homepage.aspx?Lang_Id=1http://www.sunhydraulics.com/cmsnet/GenPDF/RPEC.pdf


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Solid Model with Areas and Mass

Figure 4: General Schematic from Sun Hydraulics

From the Sun Hydraulics website catalog the three dimensional CAD model was downloaded into

SolidWorks. The solid model is shown in figure 5 was used in conjunction with figure 4, the different

attributes and components of the pilot operated relief valve were understood through these two

figures. The different areas used in the MATLAB equations to find the various forces acting upon the

balancing piston and poppet were found in SolidWorks.


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area of all three pilot stage bleed off ports, and A6 cross sectional area of all eight balancing piston

bleed off holes. The last two areas are defined by a total area subtracted from an orifice area shown as

follows: A1 cross sectional area of the balancing piston stage minus area two (orifice) and A7 cross

sectional area immediately after balancing piston minus area two (orifice). The motivation to subtract

area two from area one and seven stems from finding the forces that will be exerted on the balancing

piston. The found areas are listed in table 1.


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in the geometry. Once modeled, the mass properties feature of Pro/Engineer was used with the

material being stainless steel at a density of 0.29

3.

Mass of the poppet = M1 = 0.0048 lbf

Mass of the balancing piston = M2 = 0.0135 lbf

Figure 7: Poppet solid model Figure 8: Balancing piston solid model

Spring and Damping Coefficients

To continue with the system model, the spring constants needed to be found using the spring force on

the poppet. The spring force is an important model parameter and also defines the spring constant.The spring force is a function of the number of turns of the knob on the valve. As the knob is turned, the

spring is compressed increasing the force that is exerted on the spool, varying the cracking pressure.

1 1 5 1


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Figure 9: Cracking Pressure versus Pilot Valve Turns

Since accurate measurements were made with 2-5 turns, the spring constant calculation was

done in this region.

Fs = maximum cracking pressure (5 turns)

= (760 psi) (0.0113 in2)

= 8.59 lbf

Fi = cracking pressure (2 turns)

= (250 psi) (0.0113 in2)

= 2.83 lbf = 3 0.059 = 0.177 [12] 5.76 [13]


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Resulting:

B1 = 0.87 lbm/s

B2 = 0.81 lbm/s

SIMULINK Model

The number of turns, system pressure, and the tank pressure were modeled as the inputs to the pilot

operated relief valve. Thevalve and test systemwere constructed using SIMULINK. Several key

operations were identified as necessary steps in modeling the relief valve. These operations werelargely handled by using MATLAB functions and transfer functions. Supporting MATLAB user defined

functions were used to determine and definevariables,pressure 2, flow throughorifice 1, and flow

throughorifice 2.

A diagram of the SIMULINK model is shown below in figure 10. The number of turns is input into the

MATLAB function Spring Force, which outputs the force exerted by the spring. This force is then

input to the force to cracking pressure function, which outputs the system cracking pressure. Thecracking pressure is then subtracted from the system pressure and multiplied by the area of the frontal

side of the stage one poppet to get a force. This force is then feed into a switch which ensures that only

positive forces are sent into the transfer function. Having only positive forces, the spool can only move

to create a positive area and therefore a positive flow rate based off of our orifice equations, Equation

16 and 17. Flow-rate through the hole is represented by Q; Cd represents the coefficient for the

equation, it is usually a number between 0.7 and 1.0; is the density of the fluid, in this case hydraulic

fluid is 0.0332 lb/in3; A(x) is the area based on the displacement of corresponding spool; and Prepresents pressure, the difference in the pressure across the orifice drives the fluid flow.
http://bleedpcfc1a.mdl/http://bleedpcfc1a.mdl/http://bleedpcfc1a.mdl/http://variables.m/http://variables.m/http://variables.m/http://pressure2.m/http://pressure2.m/http://pressure2.m/http://oriface1.m/http://oriface1.m/http://oriface1.m/http://oriface2.m/http://oriface2.m/http://oriface2.m/http://oriface2.m/http://oriface1.m/http://pressure2.m/http://variables.m/http://bleedpcfc1a.mdl/


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1 22

7

= 2 [18] 117 = 2 [19]


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Figure 11: Component diagram

Analysis


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Figure 12: Flow versus square root of pressure

0 5 10 15 20 25 30 350

0.5

1

1.5

2

2.5

3

3.5

Sqare Root of Pressure (psi)

Flow

Rate(gpm)

Q = Constant*sqrt(DeltaP)

0

1

2

3

4

5


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Figure 15: Flow Rate, System Pressure and Cracking Pressure for 3 turns



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Figure 17: Flow Rate, System Pressure and Cracking Pressure for 1 turn


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Pilot Op Relief