AN OVERVIEW OF PUMPING IN PRESSURE AND VOLUME
This topic is critical to understand and is highly dynamic! It's vital to help you learn HOW to think, not WHAT to think. If we only tell you to pull levers and throttle up to XX psi, then we've only told you WHAT to do/think. Instead… we're here to help you understand HOW you should think about fundamental concepts so that you'll become more adaptive and versatile on the fire ground.
Two-Stage Pumps
Simply put, a two-stage pump is a multistage pump with two impellers, each enclosed in its own volute and [usually] part of a common body. A transfer valve exists within the pump that allows us to "transfer" or change the flow of water within the pump, allowing us to run in either Parallel or Series.
Essential Things You Need To Know:
A Multistage Pump can FUNCTION in Series or Parallel (this lays the foundation for understanding concepts like dual, tandem, relay, etc.)
You change between these two functions utilizing the transfer valve - and the main goal of the transfer valve is to regulate EWL (Engine Work Load) and maximize output in one of two directions, either volume or pressure.
There is a "break over point" in engine efficiency. The books tell us we transfer from pressure to volume once we've met ½ the rated capacity of our engine. Note: this "capacity" is not the 1500 GPM amount written on the placard on your Fire Engine because...
The manufacturer makes that recommendation with drafting (of 10 feet) in mind. In Dallas, we don't draft. We have a pressurized system allowing us to exceed our pumps' rated capacity of our pumps… which increases the break-over point when we transfer from pressure to volume.
NOTE: It is widely accepted that you only transfer between Volume and Pressure (and vice versa) at idle speed (rpm).
There is a cost-benefit to running each function (see diagram below)
Two factors determine the output of our engines; 1) water supply volume and intake pressure, and 2) EWL and our maximum RPM.
It's important to note that most Spartan Engines in Dallas have a governor that caps the EWL (in the form of RPM) at 2000-2150rpm.
When functioning in Series, Multistage pumps become force multipliers on the fire ground.
You can potentially double your pressure by utilizing the Series function from your pump. Again what determines this is your EWL because, eventually, you run out of RPM.
Cost-Benefit Analysis:
I hope you will see in this diagram that as you raise one side of the slider, you INCREASE the desired capacity. However, note that the reverse is also true.
As you raise your volume capacity, you also decrease your pressure capacity.
IMPORTANT - The EWL (Engine Work Load / available RPM) is shown in the middle of the slider. This is because it will "tip" to one side of the bar based on the "break over" point at which it becomes more efficient to run the pump in either series or parallel. This is a difficult concept, but once you see and hear it... you will never forget it.
Confused... don't worry, because it's a confusing concept. Here is a short video that explains this a bit more:
🚨 DISCLAIMER: Dallas Fire-Rescue does not endorse or promote these videos. The information herein is my best understanding of the material covered and the subsequent views expressed are my own and not necessarily those DFR. These videos are strictly for educational purposes only. It is critical that you follow your department's MOP/SOP, and talk with your Station Officer (and crew) before implementing anything you see here on The Roll Steady.
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PUMP SCHOOL W/ RICK BREWER
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