First came across this term regarding Fire Sprinkler systems. Very good explanation at Blazecut
Direct Low Pressure (DLP) System
This system is referred to as direct, which means that the activation and distribution of the agent is secured by the detection tube.
The fire suppression system operates by detecting fire and applying the agent using a detection tube fastened to a cylinder valve. The detection tube is placed in the protected enclosure and is under constant pressure. In case of fire, the detection tube degrades by the effect of fire or high temperature. When the detection tube is disrupted, the agent is released through the created hole. The detection system is independent of any electrical supply and operates solely on physical principles.
The direct system is suitable for smaller applications and combines easy installation with simple operation.
Indirect Low Pressure (ILP) System
This system is referred to as indirect, which means that it is activated by disruption of the detection tube and the agent is distributed via separate distribution tubes to the nozzles.
The fire suppression system operates by detecting heat with a detection tube connected to a valve of a cylinder. The tube is placed in the protected enclosure and is under constant pressure, whereby keeping the valve piston of the cylinder closed.
In case of fire the tube degrades by the effect thereof. The detection tube is disrupted, the pressure in it decreases and the valve piston of the cylinder opens. The fire suppression system activates and the entire extinguishing agent is released through the nozzles. The detection system is independent of any electrical supply and operates solely on physical principles.
The indirect system is suitable for larger environments or where specific application of the agent is required. This system also enables various modifications by adding optional components such as manual actuation, detectors, control panels for enhanced operation and control of the system.