The STSS Self-Testing Safety System – An Intelligent Control System for Ensuring Functional Safety (SIL3-capability) at the Highest Possible Level


The STSS was developed to test the functional safety of safety valves in gas and steam plants that are not used for prolonged periods of time in order to maintain operational readiness without interrupting or influencing plant operation. It can be universally implemented (regardless of the manufacturer of the plant) to operate each control cylinder, test the system, and prevent downtime damage by recommending frequent oil changes. This, in turn, makes it possible to significantly extend the prescribed maintenance intervals.

Danger Associated with Infrequent Use

Actuators that are infrequently used to such a degree that they deviate from normal operation are susceptible to reduced performance caused by oil resinating (chemical conversion of the oil to a sticky or rigid substance) or the silting effect occurring (jamming of the hydraulic valve pistons as a result of the accumulation of contamination particles between the piston and housing).

STSS in Hardware

The STSS comprises three main assemblies in its simplest configuration: A hydraulic valve block with two active cartridge elements that are connected in series and a matching pilot seat valve, sensor technology for pressure and position measurement, and a safety-programmable control system. Specific SIL-capabilities can be realized if demanded.

Cylinder Function Test Prevents Downtime Damage

The control system outputs alternating control signals to the pilot valves of the cartridge elements at defined intervals. The behavior of the overall system is registered and evaluated in the process (e.g. the switching movement of the cartridge valve pistons, the movement of the actuating cylinder piston rod in the elasticity range, and the pressure ratios in the operating medium at the gas or steam valve).

This system test allows conclusions about the operational readiness of an actuator without negatively affecting plant operation while ensuring that the oil in the lines is continuously exchanged.