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Water-based hydraulic systems
Water-based hydraulic systems traditionally have been used in hot-metal areas of steel mills. The obvious advantage of water systems in these industries is their fire resistance. Water-based hydraulic systems also have obvious cost advantages over oil-based fluid. First, non-toxic, biodegradable synthetic additives for water cost $5 to $6 per gallon. One gallon of concentrate can make 20 gallons of a 5% solution, so the cost of water-based hydraulic fluid actually can be less than 30 cents per gallon.
Considering the costs associated with preventing and cleaning up environmental contamination, water-based hydraulic systems hold the potential for tremendous cost savings at the plant level. Oil that has leaked already becomes a very important problem. It must be collected, properly contained. Water containing synthetic additives, however, can by dumped into plant effluent systems.
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 gallons of additive mixes with water to make 100 gallons of water-based fluid. A 50gallon container is certainly easier to handle than two 55-gallon drums, so warehousing is simpler, cleaner, and less cluttered. Transportation costs also are lower.
Other potential plant-wide savings include improved safety for workers because the water-based 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. Water is "hot" again
The oil embargo in the 1970s sparked interest in water-based fluids as a less-costly alternative to oils. Even the most expensive water additives became attractive when designers learned that one gallon of concentrate would make 20 gallons of fluid.
As oil prices gradually dropped, so did interest in water-based hydraulics. In retrospect, interest in water-based fluids centered around their cost saving potential. Most designers lost interest when they discovered that they could not just change the fluid in their systems from oil to water without making other substantial changes. They then become reluctant to accept other "disadvantages" - read substantial changes - of switching over to water-based hydraulics.
What were viewed as disadvantages were really different rules that apply to water-based hydraulic systems? Designers probably resisted learning more about water-based hydraulics because they were intimated by all the work required to lean about how to design a new system or retrofit an older system. By closing their minds to this different technology, they missed 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.
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.
There is an alternative. Propylene glycol is not toxic and is biodegradable. It costs more than ethylene glycol and is not quite as effective 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.
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.
A sealed reservoir eliminates another problem suffered by many hydraulic systems: water ingression. This addresses another misconception about water-based systems: water-based systems not sealed from the atmosphere 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 seals fluid 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 below 20 ft/sec - preferably, below 15 ft/sec in pressure lines. Velocity in suction lines, in general, should not exceed 2-3 ft/sec. Velocities in return lines should be held below 5-10ft/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 cavitations.
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. 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 minimize 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. Metal surfaces in relative motion in valves for water-based fluid are separated by bearing-type materials.
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.
Fluid leakage
Leakage continues to be a nagging problem in many hydraulic systems. 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. 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.
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.
水基液压系统
传统上水基液压系统已经应用在钢铁厂炼铁领域。 这些产业中水基液压系统的明显的优点是它们的耐火性。 而且水基液压系统在费用上也优于油基的液压系统。 首先，无毒的、可被生物分解的综合性添加剂每加仑花费5到6美元。一加仑集中可生成20加仑的5%溶液，因此实际上水基液压流体的费用可以比油基的每加仑少30分。
在工厂的水平下，考虑到相关费用、防止和清理环境的污染，水基液压系统拥有节省巨大成本的潜力。液压油的泄漏已经成为一个非常重要的问题。它必须被收集、妥善控制。不过，含有合成添加剂的水，可以倾倒入工厂的污水系统。
在工厂水平下，节省成本不停留在流体的较低成本及其处理上。因为水基液压液由十部分水和一部份合成添加剂， 5加仑添加剂与水的混合物构成100加仑水基流体。 50加仑的容器当然比两个55加仑的桶更容易处理，因此储藏更简单、更清洁、更不凌乱，运输成本也较低。
其他工厂范围下潜在的节约是为工人改善安全，因为水基液是不含毒性，并且非易燃。这些特点可以减少工厂的保险费率。泄漏的成本比清理低，因为不再需要颗粒吸收剂或吸附棉条。水基流体再次变成“热门话题”
在20世纪70年代石油禁运引发了较低成本的水基液压流体替代高昂的液压油的兴趣。当设计师们获悉，一加仑聚合物可以制造出二十加仑的流体时，即使是最昂贵的水添加剂都更有吸引力。
由于石油价格逐渐的回落，因此人们对水基液压也没有那么大的兴趣了。回想起来，对水基流体的兴趣集中在其节省成本的潜力上。当设计师发现他们不能在他们的系统中改变流体从液压油到水的状况并且也没有其他重大的改变时，他们就失去了兴趣。然后，他们不情愿的接受其他的“缺点” –了解到很大的变化-又切换到水基液压。
适用于水基液压系统的不同的规则被认为是缺点。设计师可能不愿意学习更多关于水基液压，因为他们被暗示，所有的工作需要依靠如何设计一个新的系统或改造旧系统的知识。因为他们结束了对这另外技术的思维，他们错过了除水基流体初始成本以外的许多其他的优点。现在，环境问题，增加了液压油处理成本的价格，水基液压便再次成为热门话题。
抵抗凝固
当然，水基液压系统确实在应用上有它的局限性。一个限制就是潜在的凝固。这个可能性可能是更广泛地应用水基系统，特别是在移动设备业最重要的阻碍。长壁开采法是迄今为止最大的能够充分利用水基系统的移动设备部门。地下的温度不接近水的凝点和耐火性是必不可少的条件。用于温带气候海上设备和移动设备获利于水基系统的优点，但不能保证这些设备将始终用在上述凝固温度。
不过，给水基流体加入防冻液可以使其凝固温度远低于32华氏度。用在汽车上的防冻液-乙二醇-是有毒的，是不能生物降解的，因此它在水基液压中添加防冻液将击败水基液压流体在环境上的优势。
有一个替代的方法。丙二醇是没有毒性，而且是可生物降解的。它比乙二醇花费更多，并且是不太有效的一种防冻液，因此它必须使用较高浓度的溶液。减少凝固潜力的另外两个技术是要保持流体的不断循环和在实际中使用胶管。
系统的密封
水基液压系统的两个个容易被察觉的问题是细菌的大批出没，并且很难保持适当的浓度。大气下的密封系统在控制中可容纳细菌成长。此外，如果从系统排除空气，一个抗菌剂的流体能对防止细菌的增长有一个持久的影响。
一个被密封的水箱消除许多液压系统遭受的另一个问题：水的进入。这说明关于水基系统的另一个误解：没有从大气中密封的水基系统，必须密切监察，以确保该添加剂浓度保持在在允许的范围之内。这是因为水比添加剂更容易从水箱蒸发。因此，水分蒸发导致添加剂浓度增加。当新液体添加到系统时，现有的流体样本必须采取措施，以确定一定浓度的添加剂在溶液中。这些结果显示，添加剂在流体中的比例必须补充，使流体的浓度是合适的。
与大气隔绝的密封的流体系统，实际上消除了蒸发问题。泄漏的液体是包含水和添加剂的。 所以，系统液体的量在发生变化，但浓度没有变化。 系统流体通过增加水和添加剂的一种预先混合重新补充到水箱。
特殊的考虑
如果水基液压系统没有正确的设计，他们可能更容易使水泵汽蚀。应提供通畅的通道，保持流体速度低于20英尺/秒，最好是在压力线低于15英尺/秒。在一般情况下，吸油管的速度，不应超过2-3英尺/秒。在回油管的速度应低于5到10英尺/秒。当流体再进入水箱时，较高的回油管速度可以促进发泡。零部件也应注意其规格，因为流体压力和速度急剧变化可能造成溶解气体和造成类似气蚀的损伤。
对水基的系统的重要考虑是应该明确设计主要元件为水基流体所使用，而不是修改最初供油服务的系统。油管，胶管，及配件，通常对这些油基液压系统可以相同。但是，水泵，阀，执行器和供水装置，存在着一些重大不同。例如，齿轮泵，应作出超硬合金抗磨损。泵的齿轮比一个油泵应更广泛，因为水的低粘度需要一个更大的范围，以形成一个足够的润滑膜。使用在水基系统中的缸应该有青铜做的活塞，以减少活塞和缸壁之间的磨损。弹簧或O型圈的密封用来减少活塞的渗漏。
水基液压阀
水基的流体的阀通常是加了密封件，用于分离金属零件，以防止金属-金属接触。这是因为水-甚至与润滑油添加剂-没有提供完整的润滑油膜。
水基装置用的阀的价格也有少许大于油基装置的。这可能是另一个原因，水基系统没有得到广泛的接受。本来，当设计系统时，面积较大的元件水基流体创建了一个障碍，并且更加昂贵的制造价格使水基液压阀的价格提高了3倍或更多。不过，现在阀大小和油基系统的相当。许多阀可达到美国国家防火协会标准。价格差异也较少。水基液压元件仍可能花费比油基系统高3 ％以上，但当考虑到水基系统节省成本的潜力，这样做可能会得到更多的实惠。
流体的泄露
在许多液压系统中泄漏仍然是一个恼人的问题。新的密封材料及设计，及O型圈，是解决密封装置泄漏的功能强大的武器。由于误用，安装不当，或缺乏简单的了解，泄漏的问题仍未解决。虽然在大多数的系统中没有任何理由泄漏，但它仍然出现。假设不解决泄漏，在不久的将来，水基液可以大大降低泄漏的相关费用。
内部泄漏是很浪费的。这种泄漏能能使温度升高，从而使液压油碳化。内部泄漏通常是泄露回容器的，因此这一技术转化成机械能的热，这并不是有益的工作。在一个水基液压阀运动部件中采用不锈钢阀芯聚四氟乙烯密封。因为没有间隙，所以就不存在内部泄漏。
但除了明显的和无形的液漏成本外，处理已泄漏液体已经成为一个被人关注的问题。考虑到除去或取消成本时，允许液压油进入污水厂系统成为一个昂贵的主张。实现这一清理和处置成本只会上升，并且石油价格的不稳定表明，水基液压可以成为一个利于解决环境问题比较经济的办法。 |
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