Keep NPSHₐ > NPSHᵣ At All Times Pump Running?

11/24/2025

According to NFPA 20, for any pump installation, the NPSH supplied must be at least equal to the NPSH required for the operating conditions specified.

So does the NPSHₐ need to be continuously maintained at or above the NPSHᵣ during pump operation?

During the operation of the fire pump, the liquid level of the fire water tank will keep dropping, which may lead to a situation where the NPSHₐ is lower than the NPSHᵣ (unless the fire water tank is built at a very high elevation).

Is this situation permitted?

Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe

6 Comments

Michael Newell

11/24/2025 08:26:42 am

There is an allowance in NFPA 20 to allow up to a -3psi when taking suction directly from a tank.

If you don’t have a tank you won’t want the suction pressure to drop below 0.

If you are on a city supply you want to verify that you don’t pull the city supply below their allowed minimums which is normally 20psi at the street.

Cris

11/24/2025 06:47:48 pm

So is it acceptable that NPSHa < NPSHr due to the drop in water tank level during the pump's operation? Won't this cause cavitation in the pump?

Chad

11/24/2025 08:49:03 am

Mike's answer, to the point, spot on.

Dustin Tures

11/24/2025 09:50:44 am

Design your pump and tank system to provide the necessary pressure at the end of the required water supply duration. Ensure your pump rep understands the available tank head, suction losses, and available suction head at the pump flange at this time as well, and that it can be negative; some reps don't.

Jack G

11/24/2025 11:48:08 am

Npshr is a pump property. Its a minimum suction pressure to prevent pump cavitation .
It's sometimes shown as npshr3 where reduced by 3% .
Npsh-r and A are not constants , the, are both functions of flow and temperature.
Npsh - R is a pump property.
Npsh - is a s,stem propert,.

Cris

11/24/2025 06:54:00 pm

Yes, it also indicates the required water level height in the water tank for the pump prior to operation.

Leave a Reply.

Home
Our Cause

The Blog
The Forum

PE Exam Prep
The Toolkit
MeyerFire University

Pricing
Login
Support
Contact Us

MeyerFire.com is a startup community built to help fire protection professionals shine.
Our goal is to improve fire protection practices worldwide. We promote the industry by creating helpful tools and resources, and by bringing together industry professionals to share their expertise.

MeyerFire, LLC is a NICET Recognized Training Provider and International Code Council Preferred Education Provider.
All text, images, and media Copyright © 2016-2025 MeyerFire, LLC

We respect your privacy and personal data. See our Privacy Policy and Terms of Service. The views, opinions, and information found on this site represent solely the author and do not represent the opinions of any other party, nor does the presented material assume responsibility for its use. Fire protection and life safety systems constitute a critical component for public health and safety and you should consult with a licensed professional for proper design and code adherence.

Discussions are solely for the purpose of peer review and the exchange of ideas. All comments are reviewed. Comments which do not contribute, are not relevant, are spam, or are disrespectful in nature may be removed. Information presented and opinions expressed should not be relied upon as a replacement for consulting services. Some (not all) outbound links on this website, such as Amazon links, are affiliate-based where we receive a small commission for orders placed elsewhere.

Keep NPSHₐ > NPSHᵣ At All Times Pump Running?

Leave a Reply.

ALL-ACCESS

SUBSCRIBE

COMMUNITY

YOUR POST

PE EXAM

FILTERS

ARCHIVES

PE PREP SERIES