Static pressure in a sprinkler system is a recurring topic: the “correct” value is related to the pump start-up thresholds and the maximum head (Hmax) of the main pump. Here I summarize the operational rationale outlined in UNI EN 12845, with a practical focus on main pumps and Jockey Pumps.
Table of Contents
Legal Basis and Underlying Logic
To correctly set the static pressure in a sprinkler system, refer to section 10.7.5.2 of UNI EN 12845. The approach does not start with an “absolute” number that is the same for all systems, but rather with a value relative to the pump: p (Hmax).
In other words, the static pressure “to be maintained” is determined based on the pressures at which the pumps must start (and the need to avoid unnecessary starts).
What is p (Hmax) and why does it matter?
According to UNI EN 12845, p is the head (Hmax) that the pump achieves at Q = 0, that is, when the outlet line is closed.
This point is crucial: if you do not know p (Hmax), you cannot correctly set the thresholds expressed as fractions of p (for example, 0.8p). Therefore, first identify p, then set the start-up and maintenance pressures.
Static pressure in a sprinkler system: minimum threshold with one pump
The specified operating rule is that the first main pump must start automatically when the pressure in the system drops to a value of no less than 0.8 p.
This leads to a practical consequence: at a minimum, according to the proposed reasoning, the static pressure must be > 0.8 p, so as to avoid triggering the main pump during normal minor pressure drops.
This “> 0.8p” is a basic criterion for preventing the system from operating too close to the main circuit’s trip threshold.
Start-up with two pumps: 0.8p and 0.6p
If two pumps are installed, UNI EN 12845 specifies two separate thresholds:
- First pump: start-up at a pressure of no less than 0.8 p
- Second pump: start-up at a pressure of no less than 0.6 p
This two-part threshold clarifies two aspects:
- The network must be able to “tolerate” minor variations without immediately triggering major ones;
- The second pump kicks in at a lower pressure, as the next stage.
The static pressure is therefore set to remain consistently above the first threshold (0.8p), with micro-adjustments handled by a dedicated component.
The Role of the Jockey Pump
Fire protection systems should include a service pump, also known as a Jockey Pump or pilot pump (as specified in UNI EN 12845, sections 3.49 and 10.6.2.5).
Its function in the text is twofold:
- automatically replenish small water losses
- keep the system pressurized without engaging the main pumps
This explains why, in practice, the “apparent” static pressure value downstream of the water pump station is often more closely related to the jockey pump’s shutdown logic than to the main pump’s startup threshold.
Typical start/stop settings for the jockey (and why they must be above 0.8p)
The jockey pump is equipped with a pressure-controlled circuit that is independent of that of the main pumps and is started or stopped based on the levels set on its pressure switches.
To achieve the described behavior (micro-resets without restarting the main systems), the following conditions must be met:
- The jockey’s START and STOP are both > 0.8 p
The text presents a typical approach:
- START 0.9 p
- STOP 0.95 p
Consequently, the static pressure in the network downstream of the water treatment plant is approximately 0.95 p, because it corresponds to the level at which the jockey pump stops and “leaves” the network under pressure.
Be aware of the maximum pressure limit (12 bar)
This reasoning must always be verified against the provisions of Section 8.2 of UNI EN 12845, which specifies the maximum permissible water pressure within sprinkler systems.
The text provides a clear instruction: even when the outlet is shut off (Q = 0), the pressure must not exceed 12 bar.
It is also noted that, in systems with a high vertical rise, the pressure may exceed 12 bar; in such cases, the actual values must be taken into account, and appropriate components must be selected. In practice, this means that the setting (e.g., 0.95p) must always be compared with the maximum allowable operating pressure limit.
How to Determine p(Hmax) in Practice
To find p (Hmax), the text suggests two approaches:
- obtain the Q/H curve for the main pump
- Alternatively, during a test, read the pressure on the pressure gauge located on the pump’s outlet line when the outlet is shut off (Q = 0)
Once p has been determined, the thresholds can be set accordingly: the main start threshold at 0.8p, the (optional) second threshold at 0.6p, and the jockey logic with start/stop above 0.8p.
Note on the “hydrant network”: What It Takes to Respond Correctly
The question also covers the fire hydrant system, but the text provided elaborates on the criteria by referring to UNI EN 12845 and principles typical of sprinkler systems (main pumps + jockey pumps, thresholds of 0.8 p/0.6 p, limit of 12 bar).
To answer correctly and without making any assumptions, you’ll need to tell me which references and criteria you want to apply to the fire hydrant network (or provide me with an equivalent text for that section).
Conclusion
In summary, the static pressure of the sprinkler system is set starting from p (Hmax): the first pump starts at 0.8p, and the second (if present) at 0.6p. To prevent unwanted starts, the jockey pump operates with start/stop points set at both > 0.8p (typically 0.9p and 0.95p), provided that the pressure does not exceed 12 bar even when Q = 0.
