HydraForce Insider Blog

When designing a hydraulic system, you want to optimize your design for minimal pressure drop. Savvy system designers have found that using a custom manifold with the right combination of cartridge valves is one way to optimize a system. When using this approach, it is important to correctly size your valves, drillings, and flow paths within the manifold. It is just as important to “right-size” the hydraulic hose and pipe connecting the manifold to the rest of the installation. Hose and tubing need to be the right diameter, length, smoothness and shape to handle the demands of the pressurized hydraulic flow. Undersized hose or tube can cause turbulent flow and excessive heat buildup. Over-sized hose or tube can add cost, size and weight to a system and decrease the rate of flow.

To understand what “right-sizing” means in terms of hydraulic hose and tube, you must first understand the nature of fluid and friction. Whenever fluid flows, there is a loss of mechanical energy to overcome viscous forces within the fluid. In a hydraulic system, this loss is seen as a pressure drop in the direction of flow.

Each component within the hydraulic system will contribute toward the pressure drop, i.e. cartridge valves, tubing, fittings, hoses, filters etc. This lost energy is dissipated as heat energy in the oil.

Frictional losses in pipework are mainly dependent upon:

• Length of pipe

• Cross-sectional area of pipe

• Roughness of pipe surface

• Number of pipe bends

• Velocity of flow

• Viscosity of fluid

The total allowable pressure drop of the hydraulic system must be chosen with care, as the power loss is a product of the system flow rate and pressure drop. This is an efficiency loss that has to be balanced against the cost of larger pipework/hoses and fittings. The wasted energy is dissipated as heat energy in oil, which may lead to cooling problems and shortening of the oil life.

Pressure losses in pipework will depend upon the flow condition. There are two distinct flow conditions: 

1. Laminar Flow and

2. Turbulent Flow.

Laminar flow is the condition when the fluid particles travel smoothly in straight lines, the inner-most fluid layer travels at the highest speed and the outer-most layer at the pipe surface doesn’t move, as shown in Figure 1.

Figure 1 – Laminar Flow

Turbulent flow has irregular and chaotic fluid particle motions, such that a thorough mixing of the fluid take place, as shown in Figure 2. Turbulent flow is usually not desirable, as the flow resistance increases and thus the hydraulic losses increase.

Figure 2 – Turbulent Flow

The Right Calculations

To determine the right size of hydraulic piping, you must first do the right calculations for the nature of the hydraulic flow in your system.

Osborne Reynolds discovered that the flow condition depended upon the mean flow velocity, the diameter of the pipe and the kinematic viscosity of the fluid, formula 1.

Q = Flow (m³/s)

d = Pipe internal diameter (m)

ν = Kinematic Viscosity (m²/s)

A Reynolds number of 2000 or under is deemed to be laminar flow. A Reynolds number above 3000 indicates turbulent flow.

Pressure loss in straight pipe can be calculated using Poiseuille’s equation (for laminar flow only). See Formula 2.

A more general equation used for turbulent flow and laminar flow is D’Arcy equation. See Formula 3.

ΔP = Pressure Loss (Nm^-2)

F = Pipe Friction Factor

L = Pipe Length (m)

D = Pipe internal diameter (m)

ρ = fluid density (kg m ^-3)

U = Fluid mean velocity (ms^-1)

The friction factor (f) depends upon the nature of the flow in the pipe. The most convenient form of depicting friction factors are from a Moody Diagram. However for hydraulic systems it is often assumed the pipe conditions are smooth.

A quick and easy and more common method of determining pipe/hose sizing, is to calculate the diameter size based upon recommend fluid velocities. See Table 1.

 Table 1 – Recommend Mean Fluid Velocities

 Based upon using the mean fluid velocities, the appropriate hose/pipe diameter is determined using Formula 4.

Rearrange the formula to get:

d = Diameter (mm)

Q = Flow (lpm)

V = Fluid Velocity (m/s)

This quick check calculation is useful to ensure that potential pressure drop and energy loss due to hose/pipework is not excessive when designing your manifold installation. For critical long runs of pipe or hose, the pressure losses should be checked using Poiseuille’s or D’Arcy equations as briefly discussed earlier.

Don’t let the wrong size of hydraulic hose or pipe keep your hydraulic system from reaching its maximum efficiency. Do the right calculations, and specify the right size of hydraulic hose and tubing to keep your installation at optimum working pressure.

Are You Ready to Move from On/Off to Proportional Control but your Customers Aren't?

This is an ongoing battle for most of us. There is some pretty slick new technology out there that will make most equipment more efficient, safer, lighter, smaller, and even greener, but getting the end users and industries to adapt is quite another issue.

One of the simplest moves in this area from a valve standpoint is moving from bang/bang or On/Off control to proportional control. I use the word “simplest” quite loosely here, so let me explain.

There are still lots of industries out there where Manual Levers are KING and getting operators to move to joysticks, control panels, buttons and knobs is an uphill battle. I am not sure we will ever get away from the manual levers, but for those of you whose equipment is already electro-hydraulic, moving from on/off to proportional can be quite simple.

When looking at creating custom mono-blocks or custom manifolds, we all know that quantity plays a role in the cost-effectiveness of this option. So, if you needed one manifold for on/off and one for proportional, going the custom, mono-block option is probably not feasible. For example, if the majority of your machines use on/off, while only a few select customers see the advantages of proportional, designing the machine to accommodate two different custom manifolds is just not practical. But what if you could design one custom manifold block that can be either on/off or proportional by switching out the on/off valves with the equivalent proportional valves? In other words, you could use the same manifold block for 100% of your applications even though some of those applications are on/off and some are proportional. I will review an example and discuss a little about the cost implications, but for the most part, you will see that this option is very feasible.

Let’s take a sweeper application. In most cases on/off control of the brooms is sufficient and probably an industry standard, so most operators are used to it and therefore prefer it. However, having proportional broom control offers clear advantages, such as: slowing down the brooms for certain surfaces which could extend broom life, as well as having the option of controlling broom speeds for specific debris, which would improve productivity. A very cost-effective solution can be created by designing a custom manifold that uses on/off valves for the majority of users, but has the proportional option for the progressive users just by swapping out a cartridge. (Electrical scheme notwithstanding but I will discuss this later.) So, as the sweeper OEMs push to move their customers and industry to proportional, the packaging of the hydraulic valve system doesn’t have to change, keeping overall cost to a minimum.

Below is a simplified version of the on/off circuit and the proportional circuit. The trick is that the port logic and cavity details must be exactly the same for both the on/off and the proportional valve. In this example, both valves use the VC12-3 cavity and both have the same port logic of open from port 1 to port 3 with port 2 blocked in the normal condition. And when energized, port 1 opens to port 2 blocking port 3 for the on/off, and proportioning flow from port 1 to port 2 while bypassing what isn’t needed to port 3. In this case, the valve hardware change is roughly $55.00 list per valve.

The advantage is that the end users can upgrade their equipment with a field kit from the OEM. As the industry moves toward proportional, and the mix of machines starts to change to more proportional than on/off, the OEM doesn’t have to create a new manifold. The manifold can be preconfigured so that it has commonality of parts, which means lower development cost and service costs, usage of the custom component (manifold in this case) remains consistent, longer machine production life, and so on.

To help you look at this as an option for your equipment development, here is a list of HydraForce on/off valves and their equivalent proportional valve partners having the same cavity detail and port logic:

Some of these conversions are pretty self explanatory, while other might be a bit confusing. Switching the SV08-20 with an SP08-20 is obvious. However, why would you switch from an SV08-33 directional selector to an EHRP08-33 proportional pressure reducing valve? In this case, these two valves would be used in conjunction with a pilot element. In our case the PD16–S67 would be piloted with the SV08-33 selector for the on/off version. If proportional directional control is needed, swapping out the PD with the PE metering element and using the EHPR proportional reducing valve will control pressure against the PE springs, which gives you the proportional movement of the metering spool.

In this case the additional valve hardware cost is less then $100 list.
A great feature for such a small price increase.

Another thing to keep in mind when designing a custom manifold solution for either option: the coils may or may not be identical when going from the on/off to the proportional. (My pricing comments include the coil changes.) In both of my examples, the SV12 spool valve coil was changed to the 70 size coil used on the PV’s and the SV08 coil was changed to an 06 EHPR coil. You can see how planning for adequate spacing of components on the manifold is critical. Check out our free i-Design software for easy manifold customization and configuration flexibility.

Depending on how your machine controllers are configured, a simple “patch’ download for proportional would cost virtually nothing. It’s hard to say where the costing for the electronics would fall. But doing your due diligence in the beginning and planning for this feature will definitely keep the costs down. Adding proportional electronics after the fact will be much more costly. There will, most likely, be some additional costs up front in terms of the controller, the software programming, and the input device, but how will that compare to your readiness when your customers and industry take the leap? Will you be ready?

For more in-depth electronics discussion, contact your local HydraForce expert.

Lisa DeBenedetto is a Regional Manager at HydraForce with more than 20 years of hydraulic experience. She has been with HydraForce for over 16 years. Contact Lisa