By Josh Cosford, Contributing Editor
Hydraulic pressure spikes aren’t extremely common but occur in certain circumstances, which may lead to expensive damage. Pressure spikes occur in poorly designed systems or those that are misused, and you’d be surprised by the actual cause. When they do occur, pressure spikes can damage plumbing and cause erratic operation and component failure.
Pressure spikes don’t just happen from bad luck or poorly adjusted valves. Believe it or not, pressure spikes are a result of motion and not spontaneous rises in pressure. When fluid or actuators stop abruptly, it’s the kinetic energy of the fluid or load that creates the pressure spike. The larger the load or volume of moving fluid, the larger the resulting pressure spike, within reason. Many people refer to the sudden arrest of high-volume fluid as a “water hammer.”
A system appropriately designed to handle high flow and massive loads is less susceptible to pressure spikes. High-volume systems with large diameter pipes and hoses may flow enough volume for the mass of the fluid to carry enough kinetic energy to create pressure spikes when valves close. Even more extreme is when you combine high-flow circuitry with massive loads that would prefer to keep moving with respect to Newton’s First Law.
Pressure spikes also occur when loads suddenly cease motion when physically opposed. An excavator’s bucket may suddenly smack against a large stone, and the inertia of the slewing excavator must be fully absorbed by the fluid as pressure. To be fair, excavators are designed for such incidents, but such situations occur in presses, hydrostatic propulsion systems, and injection molding machines.
Some of the above machines also experience pressure spikes as a result of pressure drops. Decompression shock occurs when a high volume of pressurized fluid suddenly releases into the exhaust path, as can be expected when large bore cylinders punch through heavy wall steel on a stamping press. The spike occurs in the return line when a high volume of pressurized fluid suddenly surges into the plumbing.
The best medicine for pressure spikes is prevention. It’s best to avoid opportunities for inertia from fluids or actuators to stop suddenly. Sometimes, machine design calls for high-speed, high-inertia operation, so controlling deceleration is the best method. Using servo or proportional valves to meter the flow during directional changes does wonders. Instead of abrupt stops, the load is decelerated using reduced flow, and when done well, it can be mostly imperceptible.
Even without expensive high-response valves, there are other methods to prevent sudden spikes. Some manufacturers offer “soft shift” solenoid valves that use spools with metering notches to prevent instantaneous flow rate changes. And, of course, most mobile-style lever valves come from the factory with metering notches, although the ultimate size is limited due to difficulties in construction and operation.
If possible, direct-acting pressure valves should be chosen for systems susceptible to pressure spikes. Compared to pilot-operated valves, which offer higher flow with less pressure drop, direct-acting valves open more quickly to relieve pressure since there is no secondary spool or poppet to operate.
Finally, if you’re experiencing pressure spikes for an unavoidable or unexpected reason, consider adding a hydro-pneumatic accumulator. Accumulators excel at absorbing energy and can nearly instantly absorb pressure spikes. The larger the spike, the more quickly the differential acts upon the bladder or diaphragm to provide a temporary flow patch for the offending fluid. Of course, they should be the correct size to absorb the maximum amount of fluid without rupturing, and the precharge pressure chosen to offer the most responsiveness.
Pressure spikes are a natural occurrence with dynamic hydraulic machines. In most cases, the spikes can be designed out or made benign in the engineering stage. When pressure spikes arise in already-commissioned equipment, there are many tools for the technician to ameliorate the systems.
