Water Hydraulics: Benefits and Limitations - Power & Motion

Cost savings at the plant level don't stop at the lower cost of the fluid and its disposal. Because water-based hydraulic fluid consists of 10 parts water and one part synthetic additive, 5 gal of additive mixes with water to make 100 gallons of water-based fluid. A 50-gal container is certainly easier to handle than two 55-gal drums, so warehousing is simpler, cleaner, and less cluttered. Transportation costs also are lower.

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Other potential plant-wide savings include improved safety for workers because the wate rbased fluid is non-toxic as well as non-flammable. These attributes can reduce plant insurance rates. Spills cost less to clean up because granular absorbents or absorbent socks are unnecessary.

Nevertheless, adding an anti-freeze to a water-based fluid can depress its freezing temperature to well below 32°F. Ethylene glycol — used in automotive anti-freeze — is toxic and is not biodegradable, so its use for anti-freeze in water-based hydraulic fluid would defeat the environmental advantage water-based fluid has. Propylene glycol is an alternative, which is not toxic and is biodegradable. It costs more than ethylene glycol and is not quite as effective an antifreeze, so it must be used in slightly higher concentrations.

Two more techniques to reduce freezing potential are to keep fluid circulating continuously and use hose where practical. Hose insulates fluid from exterior temperatures; metal tubing provides little insulation.

Sealing the system

Two more perceived problems with water hydraulic systems are bacterial infestation and difficulty in maintain proper concentrations. Sealing the system from atmosphere can hold bacterial growth in check. Addition of an anti-bacterial agent to the fluid can have a lasting effect on preventing bacterial buildup if air is excluded from the system. Sealing the system from the atmosphere also keeps out most airborne contaminants — a common cause of component failure.

A sealed reservoir eliminates another problem suffered by many hydraulic systems: water ingression. Dissolved suspended water contaminates hydraulic oil. The only detriment water ingression has in a water-based system, though, is that is alters the concentration of additive. Water ingression is still undesirable, but its occurrence is far less detrimental in a water-based system than in one using oil.

This addresses another misconception about water-based systems: water-based systems must be closely monitored to ensure that the additive concentration stays within tolerance. That is because water evaporates from the reservoir more readily than the additive does. Consequently, water evaporation causes the additive concentration to increase. When new fluid is added to a system, samples of the existing fluid must be taken to determine the concentration of additive in solution. These results then reveal the ratio of additive to fluid that must be added so that fluid concentration is correct.

With a system that sealed from the atmosphere, the evaporation problem is virtually eliminated. Fluid that escapes by leakage is a solution containing water and additive. Therefore, the quantity of fluid in the system changes, but concentration does not. System fluid is replenished simply by adding a pre-mixed solution of water and additive to the reservoir.

Special considerations

Water-based hydraulic systems can be more prone to pump cavitation if they are not properly designed. Generous porting and other passageways should be provided to keep fluid velocities less than 20 ft/sec — preferably, below 15 ft/sec in pressure lines. Velocity in suction lines, in general, should not exceed 2 to 3 ft/sec. Velocities in return lines should be held less than 5 to10 ft/sec. Higher return velocities can promote foaming when fluid re-enters the reservoir. Components should also be carefully sized because rapid changes in fluid pressure and velocity can cause dissolved air to precipitate from solution and cause damage similar to that produced by cavitation.

An important consideration for water-based systems is that major components should be designed specifically for use with water fluid, rather than modified from versions originally intended for oil service. An oil valve retrofitted for water service may work, but its compromise in performance will be obvious when compared to a valve designed for water service.

Tubing, hose, and fittings usually can be identical to those for oil systems. Pumps, valves, and actuators for water service, however, exhibit some significant differences from components for oil systems. Pump gears, for example, should be made of super-hard alloys to resist wear. A pump's gear face should be wider than that of an oil pump because water's low viscosity requires a larger area to form an adequate lubricant film. Cylinders used in water systems should have bronze-clad pistons to minimize wear between pistons and cylinder walls. Spring- or O-ring-energized seals should be used to minimized leakage across the piston.

Valves for water

Valves for water-based fluid usually are packed with seals separating metal parts to prevent metal-to-metal contact. This is because water - even with lubricant additives - does not provide the full-film lubrication of oil. In valves for oil service, lapped spools can be used because oil forms a film on metal components to keep surfaces separated. Metal surfaces in relative motion in valves for water-based fluid are separated by bearing-type materials. Moreover, because of its much lower viscosity, water can readily leak through the clearances found in non-packed valves for oil service.

Valves for water service also are slightly larger than those for oil. This may be another reason why water-based systems have not gained wide acceptance. Originally, the larger size of components for water-based fluid created a handicap when designing systems, and more costly construction inflated prices of valves for water-based fluid to three times or more that of valves for oil. Now, however, valve sizes are comparable to those for oil. Many valves are available with standard NFPA footprints. The price differential has also become less. Components for water-based fluid still may cost perhaps 3% more than those for oil systems, but this may be a bargain when you consider the cost-saving potential of water-based systems.

Cartridge valves that fit into cast, ductile-iron bodies also are available, as are lapped-spool versions of interchangeable cartridges. Special materials are used instead of seals when proportional control is needed, because seals can promote unacceptable stick-slip operation.

The spool in a valve for oil service can ride directly in the valve body. Proportional valves for water-based fluid, though often have a spool that rides in a cast sleeve instead of in the valve body. The sleeve wears because it is softer than the spool. Both sleeve and spool are hardened to RC 6-72 to reduce wear rates. Valves for water-based fluid also have longer lands to reduce leakage.

Fluid leakage

Leakage continues to be a nagging problem in many hydraulic system. New seal materials and designs, and O-ring face-seal fittings are powerful weapons in the battle against leakage. But the battle is far from over because of misapplication, improper installation, or simple lack of understanding. Although there's no excuse for leakage in most systems, it still occurs. Assuming that leakage will not be eliminated in the near future, water-based fluid can dramatically reduce the costs associated with leakage.

Internal leakage can be just as wasteful. For example, lapped-spool valves are designed to leak because the leakage creates the oil film necessary to lubricant moving parts. This leakage can carburize the oil by generating heat. Internal leakage typically is routed back to tank, so this technique transforms mechanical energy into heat instead of useful work. Using a stainless steel spool with PTFE seals in a valve for water-based fluid eliminates the need for clearance between moving components. Because there is no clearance, there is no internal leakage.

Packed-spool valves eliminate leakage and the need for pilot-operated check valves. When the valve centers to an all-ports-blocked condition, pilot-operated checks are not needed to prevent cylinder drift. If there is no port-to-port leakage, the cylinder will not drift.

But beyond the obvious and intangible costs of fluid leakage, disposing of the fluid that has leaked from a system becomes a concern. Allowing hydraulic oil to enter plant effluent systems becomes an expensive proposition when removal and disposal costs are considered. Realizing that cleanup and disposal costs will only go up, and that the price of oil is unstable suggests that water-based hydraulics can be an economical solution to environmental problems.

Accepting water hydraulics

Even the most expensive water additives become attractive when designers realize that 1 gal of concentrate can make 20 gal of fluid. No wonder, then, that interest in water-based fluids often centers around  cost saving potential. However, designers must also realize that they can't just change the fluid in their systems from oil to water without making other substantial changes.

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What are viewed as disadvantages are really different rules that apply to water-based hydraulic systems. Designers probably resist learning more about water-based hydraulics because they are intimidated by all the work required to learn how to design a new system or retrofit an older system. By closing their minds to this different technology, they may miss the many other advantages of water-based fluid beyond initial cost. Now that environmental concerns have added disposal costs to the price of hydraulic fluids, water-based hydraulics has again become a hot topic.

Fighting freeze

Water-based hydraulic systems do, of course, have limits to their applications. One limitation is the potential of freezing. This possibility is probably the most significant blockade to more widespread application of water-based systems, especially in the mobile equipment industry. Longwall mining is by far the largest sector of mobile equipment that has been able to take advantage of water-based systems. Temperatures underground do not approach the freezing point of water, and fire resistance is essential. Mobile and even marine equipment used in temperate climates could cash in one the advantages of water based systems, but there is no guarantee that such equipment always will be used in above-freezing temperatures.

Reservoir design

Most hydraulic systems are best served by a pressurized cylindrical reservoir. Additional cost is the main reason why they are not used more extensively. With a cylindrical reservoir, condensation forming inside the top runs down the sides and into the sump. By elevating one end of the reservoir, solid contaminants collect at its lowest point and can be removed through a cleanout box. The cylindrical shape also withstands internal pressure without depending on the struts and stiffeners that can provide areas for contaminants to congregate in a rectangular reservoir.

A sealed reservoir must allow the fluid level to rise and fall without allowing air to repeatedly enter and exit. Several methods can accommodate a variable fluid level, but a simple and inexpensive approach uses a breather and two check valves, each with a different spring rate.

With a sealed system, fluid level is highest at initial startup, before fluid has been pumped to the system. When the system is started initially, air enters the reservoir through a breather as fluid leaves the reservoir. After fluid has been circulated through the system and returns to the reservoir, air is not allowed to exit through the breather. Instead, the air pocket becomes pressurized. When the fluid level rises further, pressure of the air pocket eventually will reach 3 to 5 psi. At this pressure, air exits the reservoir through a check valve to avoid overpressurizing the reservoir.

Pressure in the reservoir serves the additional function of precharging the main pump. The positive pressure in the suction line prevents pump cavitation. When the fluid level drops, instead of drawing in more air, the air pocket expands, which lowers the precharge pressure. Over time, the only air in the system is that which entered initially.

Special considerations for water hydraulics

Water-based hydraulic systems can be more prone to pump cavitation if they are not properly designed. Points to consider: Porting Velocity Sizing Components Porting and passageways should be provided to keep fluid velocities below 20 ft/sec — preferably, less than 15 ft/sec in pressure lines. Velocity in suction lines should generally not exceed 2-3 ft/sec. Velocities in return lines should be held less than 5-10 ft/sec. Higher return velocities can promote foaming when fluid reenters the reservoir. Components should be carefully sized: rapid changes in fluid pressure and velocity can cause dissolved air to precipitate from solution and cause damage similar to cavitation. Major components should be designed specifically for use with water fluid, rather than modified from versions originally intended for oil service.Tubing, hose, and fittings usually can be identical to those for oil systems. Pumps, valves, and actuators for water service, however, exhibit some significant differences from components for oil systems.

Control Valve: How It Works | Advantages & Disadvantages

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What is a control valve?

Control valve is used to regulate the fluid flow by changing its size or course as directed by the signal from a controller. It helps in directly controlling the flow rate and thus helps in regulating the other vital processes quantities such as temperature, liquid level/flow, and pressure. It is also refereed as ‘final control element’ in automatic control terminology.

In other words, control valve is used to control the flow, pressure, liquid level, and temperature of the system by completely or partially opening or closing it based on the signals received by the controllers. The electric, hydraulic and pneumatic actuators automatically controls the opening and closing of the control valve while the positioners control the closing and opening of these actuators.

How it works?

The process plant comprises of numerous control loops in order to deliver consistent quality products. These control loops have a set pressure, temperature, flow, level in order to maintain the required operating range. Each of these control loops experience internal disturbance and these disturbances are measured by sensors and transmitters. The information collected is then processed by the controllers to decide what should be done to rectify these load disturbances. Once the data collected is analyzed, measured, compared and calculated then a controlling element is implemented. This is where the control valve comes into the picture and work to reduce these disturbances.
Thus, the control valve working principle revolves around the manipulation of the flowing fluids such as water, gas, steam or chemical compounds to ensure minimizing the load disturbance and regulating the process variable to the closest value of the desired set point.

Values categorization: It’s advantages and disadvantages

On the basis of stem movement, valves can be broadly classified as linear or rotary type. Furthermore, based on the types of services it offers, it can be classified as:
For ON-OFF Services – Gate, Diaphragm Ball, Plug, Butterfly valves.
For THROTTLING Services – Globe, Diaphragm, Butterfly, Pinch
For NON-REVERSE FLOW – Check valves

Gate Valves

Gate valve comprises of flat barrier which is adjusted to effect the area of flow. It is best suited to regulate the high temperature and pressure for variety of liquid flow and has an on-off application. This design or body is used for the valves that manages the automatic emergency shut off and also for the hand-operated valves.

Advantages of Gate Valve

• • It has a good Closing feature
  • Can be used in either ways in circuit
  • It provides laminar flow and hence the loss of pressure is minimum

Disadvantages of Gate Valves

• • • • Closing and opening feature is not very prompt.
      • Assembling these valves and then erecting it, starting it and maintaining it requires large space.
      • Prone to leakage when system operate in high temperature that fluctuates periodically.
      • It creates vibration

Globe Valve

Globe valve is used for throttling purpose and the term ‘globe’ implies to the exterior shape of the valve. The globe valves have a stem that adjusts itself linearly in up-down motion in order to change the position of the plug. It has a short stem travel with high seating capability and large pressure drop and high flow controllability.

Advantages of Globe Valve

• • • • Has a good full closing feature.
      • Faster opening and closing time in comparison to gate valve as the strokes are smaller.
      • Has a good throttling feature.
      • Can work as a stop check valve with little change in shaft disconnection.

Disadvantages of Globe Valve

• • • • Higher pressure loss in comparison to gate valve.
      • It requires actuator that has a larger torque in order to close under high pressure.

Plug Valves

Plug valve comprises of plug, body, and cover and is used for on-off services basically in refinery, chemical industry, and petrochemical. Normally small in size, require less headroom and available in wide range of materials. They provide tight shut off, quick opening and low pressure drop

• • • • It comprises of few parts and has a simple design.
      • Opens and closes easily.
      • It can be maintained and repaired easily at the place of operation.
      • They have low flow resistance and has a reliable leak-proof feature

Disadvantages of Plug Valves

• • • • It creates high friction due to fast movement and hence requires large amount of force.
      • Valve transitions usually narrows flows due to conic plug.
      • Expensive than the ball valves.

Ball Valves

Ball valve is used to control pressure and flow of the corrosive fluids, normal liquids and gasses and also of slurries. It also regulates high temperature and pressure.

Advantages of Ball Valves

• • • • Offers leak-proof service.
      • Fast open and close service.
      • In comparison to gate valves has a very small dimension.
      • It is light weight in comparison to gate valves.
      • Has a multi-way design flexibility to reduce the number of valves required.
      • Provides reliable and safe service under high temperature and pressure conditions.
      • Requires less force to control in comparison to gate and globe valves.

Disadvantages of Ball Valves

• • • • Not an apt solution for permanent throttling.
      • It can cause leakage, abrasion and more such problems because the residual fluids collide and sticks to the base.

Diaphragm Valves

Diaphragm valve is used for corrosive fluids under low temperature and pressure conditions. The diaphragm valve varies the resistance to flow by deforming one surface by the force from the valve stem.

Diaphragm Valves Advantages

• • • • The diaphragm completely keeps the working parts in isolation from the process fluids.
      • Simple in construction, easier to operate and maintain.
      • It has hassle-free operation with simple construction and ease of operation.
      • It can be used for opening, throttling, and closing, purposes.
      • Based on the internal sheathing of the body, it provides good chemical resistance.
      • No shaft leakage as the fluid remains isolated from the bonnet group.
      • Suits heavy chemical, chemicals, and radioactive fluids.
      • It can prevents microbial contamination to the system as the fluid remains isolated from the bonnet group and thus can be used in food, pharma, and beer sector.

Disadvantages of Diaphragm Valves

• • • • Complete discharge of the pipeline is prevented as the valve transition has sets.
      • Operating temperature and pressure has limits based on the diaphragm material.
      • There is a limitation to the hydrostatic pressure that can be applied to the diaphragm.
      • It has limited dimensions of diaphragm valves.

Conclusion:

For more information on various control valves specifically required for your industrial plants / business, Contact Flowspec and we will be happy to answer your questions and give you all the details you need.

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