Common problems with hydraulic equipment

Overheating ranks No.-2

In the list of most common problems with hydraulic equipment. Unlike leaks, which rank No.1, the causes of overheating and its remedies are often not well understood by maintenance personnel. Why do hydraulic systems overheat? Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat.

Concerns about Hydraulic System Overheating

When hydraulic systems overheat several things can occur that can damage the system.

(1) Breakdown of the hydraulic oil producing “sludge”,

(2) Deterioration of hose lining releasing rubber particles into the system,

(3) o-rings and seals become less pliable and begin to leak, and

(4) Increased formation of acids that can begin to corrode components.

If you are experiencing hydraulic system overheating, be sure the hydraulic oil viscosity is correct for the ambient conditions. Correct any overheat problems as soon as possible.

The major causes of heat in a hydraulic circuit are:

· The inside diameter of the hydraulic “return” hose is too small.
· Hydraulic “pressure” and “return” hoses are too long.
· Hydraulic reservoir is too small.
· Hydraulic oil reservoir is low on oil.
· Internal parts in the tool / cylinder are worn  beyond part tolerance creating oil bypass.
· No oil Cooler.
· Hydraulic power source exceeding system relief pressure.

Inefficient designs, poor plumbing practices and the overriding idea to make it a cheap as possible will produce 8 out of 10 hydraulic systems with a heat problem.

Hydraulic fluid temperature - How hot is too hot?

Most hydraulic systems should operate between 48°C and 57°C under normal operations. This temperature is the average temperature taken at the oil reservoir, and higher temperatures can be seen at other locations in the circuit. Oil temperatures above 75° C damage most seal compounds and accelerate degradation of the hydraulic fluid. The fluid temperature is too high when the fluid viscosity falls below the optimum value for the hydraulic fluid system components. This can occur well below 75° C, depending on the fluids

Generally, the system should not allow the oil to get more than 15°C over that of the surrounding (35°C) air. HTMA recommends a heat dissipation capacity of at least 5 KW for 40 lpm Circuit. Using any ISO32 oil, based on 55°C, every 8°C increase in oil temperature reduces the life of the oil by 50%.

Why use an air-cooled heat Exchanger/cooler

Air-cooled heat exchangers are generally used where a process system generates heat which must be removed.  Most of the heat must be dissipated somehow. One of the simplest ways is to use the ambient air. Air-cooled heat exchangers (often simply called air-coolers) do not require any cooling water from a cooling tower.

APEX

Air-cooler are usually used when the outlet temperature is more than about 4-6 deg. C above the maximum expected ambient air temperature. They can be used with closer approach temperatures, and often become inexpensive compared to a combination of a cooling tower and a water-cooled exchanger.

Maintaining the correct system temperature is important to ensure optimum oil viscosity in a hydraulic system. In many hydraulic systems only 50% of the consumed energy is left to do its designated work. The remaining 50% is lost by friction, pressure drops and lost due to interruptions in flow conditions.

Lost energy results in temperature unacceptably high for the oil as well as for the hydraulic system components. The vital physical characteristics and the lubricity of the oil are considerably reduced.

The Air cooled oil cooler provides reliable temperature control

The Air cooled oil cooler reduces the difference between consumed and input energy providing reliable temperature control of oils and a wide range of other highly viscous fluids. The correct type of cooler and oil means a system that operates properly with maintained oil viscosity.

 Inherent advantages of oil cooler

• Hydraulic system life extended by 3-5 times.

• Long lasting oil durability.

• Reduced internal potential for leaks.

• Prolonged lubricating qualities.

• Maintained hydraulic efficiency for the entire working cycle.

The negative aspect of a cooling tower

The cooling water is directly open to the environment. Airborne particulate contaminants are washed out of the air by the water flowing over the tower. The water also absorbs oxygen and other gases, including products of air pollution.

The evaporation process causes the minerals that were initially dissolved in the water to be left behind as fine, highly abrasive particles. It also causes the mineral concentration of the remaining water to increase. As a result, cooling tower water quickly becomes highly contaminated water that causes fouling, scaling, corrosion and erosion of heat transfer surfaces. These detrimental effects can increase maintenance costs as well as incur unscheduled equipment and process downtime and loss of productivity

The negative aspect of Shell & tube type heat exchangers  

-Laminar flow of water in the tubes results in falling of heat transfer surface. A hard layer gets deposited inside the tubes narrowing passage. This layer of deposits acts as insulter and does not allow the heat to be transferred from oil to water.

-When water passes through the tube the highest velocity is at the cavitations. The velocity near the walls of the tube is reduced to almost zero; this creates a stagnant water film near heat transfer area which is a bad conductor of heat.

-When leakage starts in shell & tube type heat exchanger water mixes with oil of hydraulic system resulting in further damage to the system.

The negative aspect of air cooled oil cooler

- In aggressive environments ambient air may contain dust particles that will increase air fin clogging coefficient.

- Operating coolers at high altitudes will reduce heat dissipation.

- Oil temperature is more than about 4-6 deg. C above the maximum expected ambient air temperature.

Basic DON’TS for cool oil control

DON’T – Rely on a large tank to control oil heating

DON’T – Set relief pressure too low. For oil pulsing type application pressure peaks may run up to 350 PSI over gauge pressure, popping the relief and causing heat as well as low performance.

DON’T – Pump more oil than the application should use. Avoid flow controls if possible. It is best to size the pump for desired flow volume.

DON’T – Use heavy oils such as 30W or 10W30 engine oils. These will cause high losses in lines and add to back pressure and heat.

DON’T – Run return oil through control valves or other series circuits.

DO the following to reduce heat generation

1. DO - Operate pumps at moderate speed – gear pumps usually generate less heat (and

Are less prone to cavitations) at speeds of 1,000-2,000 rpm. If this speed range can be used, pick a pump size to supply desired flow in this speed range. Power take-offs are available at various speed ranges.

2. DO - Use generous line sizes – Especially on pump suction and return from tool to tank.

3. DO - Use oils in 150-200 SSU range, with high viscosity index if a range of weathe Conditions are encountered.

Provide good cooling for hydraulic oil

-Use an air-to-oil cooler of maximum size for space available.

-For oil pulsing type application, use off line air to oil cooler

Disclaimer While these recommendations for AIR COOLED OIL COOLER requirements are technically sound, it is not intended that they be considered the only method for OIL COOLING. Neither should these recommendations be interpreted as superior to or standard that would necessarily be preferred in lieu of an engineer’s design for aparticular application or system. These recommendations originate from the collective experience of leading technical personnel in the hydraulic tool industry, but must, due to the nature of the responsibilities involved, be presented only as a guide for the use of a qualified designer or engineer. Thus, the APEX PUMPS LIMITED disclaims any responsibility for damages arising out of the use,application, or reliance on the recommendations and information contained herein by designers, engineers, or by users of hydraulic tools and systems.