Balance Linear Spool Valve Alternative for HP Water - ??

[oceanman]

I would like suggestions for an alternative to the use of a linear spool valve for high pressure water flow control. The valve needs to direct seawater at a pressure which can alternate between a few psig to about 1000 psig. The flow rate is up to 200 gallons per minute. The linear spool valve appears to have been usede for this sort of application before, but I cannot use it in this case. However, I cannot use it in this case. I need to cycle the valve about once every two seconds or so. It should not be too large and should not require too much maintenance with the almost constant cycling. A pressure balance valve seems to be a good idea, but I do not know this types exist, besides spool valves. Some water leakage could be acceptable, so ideas using hydrodynamic seals may be OK.