The hydraulic power unit supplies hydraulic fluid to the entire control system. It requires fresh water, soluble oil, glycol (for freeze protection), compressed air and electrical power for operation. Two small reservoirs contain the soluble oil and glycol which are automatically blended with fresh water to make up the hydraulic fluid which is then stored in a large reservoir known as the mixed fluid tank.
Since the control system is an 'open' one in that the used hydraulic power fluid is vented into the sea, the type of soluble oil used must be completely biodegradable. Additives to prevent bacteria growth and to inhibit corrosion are also frequently included in the mix water.
The soluble oil reservoir has a capacity of at least 110 gal whilst the mix fluid tank should be capable of holding sufficient fluid to charge the system accumulators from their pre-charge condition to their maximum operating pressure. All the tanks are fitted with sight glasses and a low-level alarm system which activates a warning light and horn on the control panels.
The proper mixing fluid ratio is maintained by air operated hydraulic pumps, a water pressure regulator, a double acting motor valve and a water flow rate indicator. A reservoir float switch is used to control operation of the mixing system in order to maintain the correct level of fluid and to ensure a continued supply for the control system.
Water/additive concentrations can be adjusted by setting the mixing pump to run at the appropriate rate. A minimum rig water supply pressure of 25 psi is typically required for the correct operation of the mixing system and to provide a fluid supply at least equal to the rate at which mix fluid is drawn from the tank by the high pressure pumps.
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