Film temperature	Temperature of the valve surfaces that are in contact with the heat transfer oil at which the oil starts to disintegrate
Cavitation	Due to high speeds of the medium in the narrowest section of the valve, local underpressure occurs. If this pressure drops below the medium’s boiling pressure, cavitation occurs (steam bubbles), possibly leading to material removal (abrasion). Also, when cavitation starts, the noise level increases abruptly. Cavitation can be avoided by limiting the pressure differential across the valve as a function of the medium temperature and the prepressure. For more detailed information, refer to "2.13 Cavitation", page 56
Selection of valve characteristic	Certain types of Siemens actuators are equipped with DIL switches for the selection of a linear or an equal-percentage valve characteristic. The objective is to linearize the volumetric flow through the consumer and the valve
Closed circuit	The medium circulates in a closed hydraulic system with no contact to the atmosphere
Open circuit	The circulating medium is in contact with the atmosphere, that is, the hydraulic system is open to atmosphere (e.g. cooling towers with open tanks, or showers). Hence, the system can absorb oxygen from the surrounding air, which can lead to rust; in addition, more attention is to be paid to cavitation; for more information, refer to "2.13 Cavitation", page 56
Control stability	The stability of a closed control loop depends on the degree of difficulty S of the controlled system and the circuit amplification V0. For more detailed information, refer to the Siemens brochure "Control technology" (ordering no. 0-91913-en)
Return temperature TRL	Temperature of the medium at which it returns from the consumer to the heat or cooling source
Gravity circulation	The density of a medium depends on its temperature. If a medium is hot in one place and cold in another, it starts to circulate due to different densities
Volumetric flow V	Volume of a medium that passes through an opening for a certain time
Flow temperature TVL	Temperature of a heating or cooling medium at which it leaves its source to enter a hydraulic circuit
Selection of acting direction	Certain types of Siemens actuators are equipped with DIL switches for selection of the operating action of the respective valve (push to open, pull to open, normally open, normally closed). The objective is to drive the valve to the fully open or fully closed position should a power failure occur, depending on plant requirements
Forced control	If forced control is demanded, no consideration is given to any other control command. For example, if there is risk of frost, more heat is supplied to prevent freeze-ups