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Fire Pump Test Line Flow Meter Sizing: Venturi vs Annubar Sizing for Inspection Lines
Quick Answer
For DN100 to DN250 fire pump test lines flowing 50 m3/h to 600 m3/h, a Venturi tube usually gives better repeatability and lower uncertainty than an Annubar under NFPA 20 test conditions. Choose an Annubar when the existing header has no room for a Venturi spool or when the budget is limited. Pair either element with a 50 to 200 kPa differential pressure transmitter from Silver Instruments.
Fire pump test lines do not run continuously. They run during weekly or monthly tests and annual flow verification. The flow meter sizing decision is about repeatability, straight run, installation fit, and inspection acceptance. We have seen this on customer sites in Vietnam, Saudi Arabia, and Chile many times.
What Fire Pump Test Lines Demand
A fire pump test line branches off the main discharge header before the check valve. It can be DN80 on a small electric pump or DN300 on a large diesel pump. The flow meter proves that the pump can deliver its rated duty point. NFPA 20 asks for flow measurements at no flow, rated flow, and overload flow points. The meter does not need billing grade accuracy. It must be stable and repeatable because local inspectors and insurance engineers read the same curve every year.
Most test headers are short. The space between the test header valve and the drain point is often 2 m to 5 m. That short straight run is the real sizing problem. A Venturi tube usually needs less upstream straight run than a single point Annubar because a Venturi is a full bore device with defined geometry. Annubar devices average multiple velocity points but still need 7 to 15 pipe diameters upstream depending on the fitting arrangement.
Venturi Sizing Logic
A Venturi tube is a machined or fabricated spool with a convergent inlet, a cylindrical throat, and a divergent outlet. Sizing starts with the pipe inside diameter D. You then pick a throat diameter d and a beta ratio equal d divided by D. For fire pump test lines we recommend beta from 0.40 to 0.65. A beta above 0.70 gives low pressure loss but a very small differential pressure at low test flows. A beta below 0.40 gives a strong DP signal but high permanent loss.
Example. A DN150 fire pump test line with an inside diameter of 154 mm handles 180 m3/h at rated duty. We specify a Venturi throat diameter of 92 mm. Beta is 0.60. The calculated differential pressure at 180 m3/h is 62 kPa. At 50 m3/h the DP falls to 4.8 kPa. A 0 to 100 kPa DP transmitter still reads this value with acceptable turn down. The permanent pressure loss at rated flow is around 0.18 bar. This is acceptable for a pump discharge test line.
Use ISO 5167 3 or ASME MFC 3M for the discharge coefficient. The coefficient is usually 0.984 to 0.995 for a machined convergent section. Always use the actual pipe inside diameter, not the nominal bore. We have seen incorrect sizing when someone uses DN100 as exactly 100 mm. A Schedule 40 DN100 pipe has an inside diameter of 102.3 mm. That difference shifts the DP by more than 4 percent.
Annubar Sizing Logic
An Annubar is an insertion averaging pitot device. It senses velocity at multiple points across the pipe. Sizing uses the pipe cross sectional area, the Annubar blockage factor, the probe K factor, and the density of water at the test temperature. The K factor changes by model and by blockage ratio. A typical K factor is 0.60 to 0.65 for a diamond shaped probe.
Annubar is cheaper than a Venturi spool and installs through a hot tap or a weld o let. The permanent pressure loss is low in a large pipe because the probe body blocks only a small part of the cross section. At 180 m3/h in a DN150 pipe, a typical Annubar produces around 0.04 bar permanent loss. That is lower than the Venturi example above. But the insertion device can lose on repeatability and straight run demand. We have seen a 6 percent reading shift on a test header in the Philippines after a contractor installed the probe at 14 degrees off the flow axis.
Straight Run and Inspection Acceptance
Upstream straight run is where most projects fail. A Venturi tube installed per ISO 5167 usually needs 10D to 15D upstream after a single elbow and 5D downstream. An Annubar may need 10D to 20D upstream if the flow exists after two elbows in different planes. Fire pump test headers often have a test valve immediately before the flow meter. That valve creates swirl and a non uniform velocity profile. A
Here is the thing. Inspectors in Dubai and Jakarta often accept a Venturi tube without field calibration because the geometry is fixed and traceable. They question an Annubar reading if the straight run is short or if the insertion depth is not documented. The project engineer may pick Annubar to save capital cost but spend more time on calibration and site verification. That cost rarely appears in the CAPEX sheet.
Transmitter and Readout Pairing
Both flow elements need a differential pressure transmitter. We recommend a Silver Instruments SDP series DP transmitter with a 4 to 20 mA HART output. Select a range of 0 to 100 kPa for DN150 and 0 to 250 kPa for smaller pipes or higher flows. The transmitter should have a 316L stainless steel diaphragm and an accuracy of plus minus 0.075 percent of span or better. Wire the DP transmitter to a Silver Instruments paperless recorder. The recorder stores test points at 1 second intervals and produces a curve for the building owner or insurance review.
Do not forget to square root the DP signal before display. Some fire pump controllers accept a 4 to 20 mA linear signal and treat it as flow. That creates a large error at low flow. The correct flow output is the square root of the DP if you use a Venturi or Annubar without a multivariable DP flow computer.
Final Sizing Recommendation
Most engineers skip this part. They pick a flow meter size based on the pump rated flow only. That is not enough. Size the Venturi or Annubar for the minimum test flow as well. A fire pump test may start at 0 m3/h and step to 150 percent of rated flow. The DP transmitter must read the minimum stable point with enough accuracy. If the transmitter range is 0 to 200 kPa and the minimum DP is 0.5 kPa, the low end error can be 3 percent or more.
For a new fire pump skid with DN100 to DN250 discharge, specify a Venturi tube. For an existing test header with no spool piece, choose an Annubar only if you have at least 12D upstream straight run after the test valve. Send us your pipe size, flow range, pressure, and temperature when you need a sizing sheet. A Silver Instruments engineer will return a DP range, beta ratio, and installation recommendation by email or WhatsApp.
FAQ
Question: What flow meter size is best for a DN150 fire pump test line?
Answer: A DN150 Venturi tube with a throat diameter around 92 mm and a 0 to 100 kPa DP transmitter covers 50 m3/h to 200 m3/h well. If the line is existing and short, a DN150 Annubar may fit but it needs careful straight run and flow profile check.
Question: Which has lower permanent pressure loss, Venturi or Annubar?
Answer: A large diameter Annubar often has lower permanent pressure loss than a Venturi with a small throat. But permanent loss is not the only acceptance factor. Repeatability and straight run requirements matter more in fire pump test lines.
Question: Can I use a magnetic flow meter on a fire pump test line instead of a Venturi or Annubar?
Answer: Yes. A full bore electromagnetic flow meter works well for water fire pump test lines and has no moving parts. But some fire pump test standards or local authorities prefer a mechanical DP element because the geometry is inspectable. Silver Instruments supplies both electromagnetic flow meters and DP transmitters so we can compare both options for your exact pipe size.
Question: What upstream straight run do I need for a Venturi tube in a fire pump test header?
Answer: Plan for 10D to 15D upstream and 5D downstream after a single elbow. If the test header has a gate valve immediately before the meter, add a flow straightener or use a Venturi instead of an Annubar.
Question: How do I size the DP transmitter for fire pump test flow?
Answer: Calculate the maximum DP at 150 percent of rated pump flow. Choose a transmitter range that places this value between 70 and 90 percent of the upper range limit. Then check that the minimum test flow gives at least 1 percent of range for stable reading. For most fire pumps, a 0 to 100 kPa or 0 to 250 kPa range works.
Need a sizing check for your fire pump test line? Call Silver Instruments at Tel +86-25-68650347 or WhatsApp +86-25-52155837. WeChat is +86 15365082610. Send us your pipe inside diameter, flow range, inlet pressure, and available straight run. We reply with a Venturi or Annubar sizing sheet and DP transmitter model.
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