Prologue

EHC fluid is expensive, so it makes sense to keep the fluid in the best possible condition. This does not always work out to be the case. EHC fluid is used in steam turbine control systems that govern, among other things, the operation of a trip-throttle valve. This valve shuts of the steam supply to the turbine in case of a threat of over speed.

Figure 1: Gimple Trip-Throttle Valve

What happens to EHC fluid with normal use?

When you read the many engineering and lubrication forums on the Internet, you will find that the following problems occur most often:

• High solids contamination
• High acidity
• High phosphorus content
• High values of magnesium (Mg), calcium (Ca), sodium (Na), aluminum (Al), tin (Sn), and potassium (K)
• Sludge and gel formation
• Filter clogging
• High moisture content
• At or close to relative saturation point
• Free water is dropping out
• Discoloration

That is where the “sharks” who have been circling move in. The sharks are the equipment manufacturers who have been trolling these forums, waiting for a chance to give you “unbiased” advice to use their equipment to solve all the above mentioned problems. They offer “bandages” while ignoring the advancing disease. What is the root cause of the disease? It is neglect; nothing more, nothing less.

As with all oil types, phosphate esters—in the operating environment of a steam turbine—are a discrete system like blood in the human body. That means one problem can affect or create several other problems in the same system. That is why you must consider the entire phosphate ester system as a homogeneous system at equilibrium. The presence of one contaminant will disturb this equilibrium. It boils down to carefully maintaining the equilibrium between all the variables. For instance, moisture is the most prevalent contaminant. Its presence can progressively lead to the following contaminants:

• Hydrolysis—an autocatalytic hydrolytic degradation—generates phosphoric acid (autocatalytic means the products of hydrolysis further catalyze the hydrolytic process)
• This increases the total acid number (TAN)
• This causes increased corrosion of metal surfaces
• Corrosion creates rust particles, increasing the particulate content
• Increased particulate content and sludge creates more sludge, varnish, and gel
• This will clog the filter

There are documented cases where a trip-throttle valve was rusted in an open position causing the turbine to over-speed and self-destruct. The replacement cost in one particular case was US$13 million for a new turbine and US$100 million in lost production. The sad part is that a US$50,000 purification system could have prevented this incident that was created by neglect and exacerbated by management indecision to purchase the purification system. The general manager of the refinery was re-assigned and never heard from again.

What was this system that could have prevented a US$113 million incident? It was not a fuller's earth adsorption system. It was not an ion-exchange system. Nor was it like any other bandage solution currently on the market. It was an Allen Minivac, a portable skid-mounted vacuum distillation/filtration system specifically designed for small- to medium-sized turbine control systems containing EHC fluid.

Figure 2: Allen Minivac Flow Diagram

This fully automatic programmable-logic-controlled system operates continuously to remove the following contaminants:

• 100% of free and emulsified water
• moisture down to 2-6 wppm
• solids to less than 0.5 microns
• all varnish particles by adsorption on treated cellulose fibers
• 100% of entrained and dissolved air and other gases
• an optional fuller's earth adsorption filter can be added

If you are lucky enough to have an Allen Minivac, how do you make it work for you?

How the Allen Minivac System will keep your EHC fluid clean and long-lasting

• By removing the free, emulsified, and dissolved water the major contaminant is removed and kept to a minimum
• No more moisture means no more hydrolysis
• No more hydrolysis means no more acid creation
• Solids and sludge are removed and no longer accumulate
• Corrosion particles are removed and no longer pose a danger
• Varnish particles are adsorbed onto the treated cellulose filter fibers

Figure 3: An Allen Minivac

Of course this will occur if your EHC fluid has not deteriorated beyond salvation. What you do before you hook up the Allen Minivac is you take a sample of your EHC fluid and have a laboratory analysis done.

Figure 4: The Levels at Which EHC Fluid is Beyond Recovery

Fluid water content must be maintained at less than 10 wppm total water. This can only be done consistently by a vaccum distillation system.

The table shown above indicates at what level the EHC fluid is beyond recovery. The control system will then need to be flushed with new EHC fluid and the Allen Minivac must be connected and put in operation to keep contaminants at the level of new fluid.

Conclusion

The major drawback of using ion exchange, activated alumina, or fuller's earth cartridges by themselves is that the acidity can reach uncontrolled levels very quickly and—in all cases of ion exchange, activated alumina, or fuller's earth—metal salts will leach out into the fluid, thereby forever destroying any chance of recovery. In general, operators will not be vigilant enough to prevent run-away acidity. When faced with these conditions, the Allen Minivac quickly becomes the most cost-effective solution for maintaining in good condition.