The HSPO normally open cartridge type poppet valve can be used as a pilot operated check valve, a directional control valve (one or more cartridges can be used to provide 2-, 3-, or 4-way functions), or a prefill valve.

The main spool (poppet) is held open by a tension hook type spring suspended between the bonnet and the spool. Opening and closing of the valve is a function of the force balance in three areas: diameter "X" (port 3), diameter "X-Y" (port 2), and diameter "Y" (port 1). Pilot pressure acting on the top of the main spool tends to close the poppet.

Port 1 and 2 areas are smaller than 3, but if a higher pressure is present at those ports it may cause the poppet to open.

Availability of two (different) ratio poppets (spools) and springs provides a variety of closing/cracking (opening) ratios. The valve is constructed of steel parts, operating parts are hardened and ground as required. The cartridge is designed for easy service or field repair.