Save Money, Space, and Weight by Better Understanding Pump Rating Versus Performance

This document will take a closer look at pump ratings and capabilities. While some pump models will be discussed for reference, the purpose of this document is not to look specifically at any one pump  manufacturer. Furthermore, the flow rates referenced in the document can vary from apparatus to apparatus depending on the specific plumbing configuration.

In the fire industry there are three primary pump manufacturers: Darley, Hale, and Waterous. Each manufacturer has a variety of pump offerings that have specific features/benefits for different applications. The focus of this document is to look at the different capabilities between what we will refer to as a full body pump and a pedestal pump. The primary difference between the two pumps is that a “full body” pump comes as a full cast package with both an intake and discharge manifold already built into the pump. The “pedestal” style pump comes as just the pump volute, and the intake/discharge manifolds are either purchased separate, or constructed by the apparatus manufacturer as the pumphouse is constructed. Below is an example of each style pump for the specific manufacturers.

Each of the aforementioned pumps is available in a variety of ratings, typically the full body pumps are available from 1250 gpm to 2000 gpm, while the pedestal pumps are typically available from 750 gpm to 1500 gpm.

The ratings of fire pumps is defined in NFPA 1901 chapter 16. For fire pumps with a rating of less than 3000 gpm the pump must be able to deliver the following capacities:
  • 100% of rated capacity at 150 psi net pump pressure.
  • 70% of rated capacity at 200 psi net pump pressure.
  • 50% of rated capacity at 250 psi net pump pressure.
In addition, the pump must be capable of pumping 100 % of the rated capacity at 150 psi net pump pressure from draft through 20’ of suction hose with a strainer attached. There are also detailed requirements for the altitude, water temperature, hose sizes, lift, etc. The important part of this requirement is the fact that the rated capacity must be obtained from draft.

There are several other tests that apply, but for the purposes of this document, the flow ratings are the critical part of the test.

As with testing, there are several factors that go into the final “rating” of the pump, but to simplify things the two main factors are the pumps capabilities from draft, and the number/size of discharge outlets. When looking at a fire pump rating NFPA only considers flow on discharges 2-1/2” and larger, and the flows are very conservative. Table 16.7.1 of NFPA defines discharge rates by outlet size:

Based on this rating chart, typical preconnected discharges like crosslays, speedlays, front bumpers, and hosebed discharges, sized with 1-1/2” fittings, do not count toward the overall pump rating. A typical pumper today has two (2) driver side 2-1/2” discharges, one (1) 2-1/2” officer side discharge, one (1) LDH (5”) officer side discharge, a 3” deck gun, and at least one (1) 2-1/2” preconnected handline. That discharge package would total up as follows:

There is some debate in the industry about the “LDH” discharge, some manufacturers and experts would argue that a 3” discharge valve with a 5” Storz fitting should only be rated as a 3” outlet, while others use the final termination point to determine that the discharge should be counted as a 5” outlet. For our purposes here we are counting a 3” discharge valve with a 5” Storz fitting as a 5” outlet.

As previously mentioned the NFPA ratings are very conservative. The above chart showed that a 2-1/2” outlet is rated at 250 gpm, through practical testing we’ve shown that a 2-1/2” discharge outlet plumbed off the LDH port of a full body pump is capable of flows up to and over 1500 gpm in reality.
As previously mentioned, most full body pumps are rated anywhere from 1250 gpm to 2000 gpm, it’s important to note that all of these ratings utilize the same pump. The physical pump does not change for these ratings, it just comes down to the number of discharges. Most standard pumpers have enough discharges to be rated all the way up to 2000 gpm, so in the end it really comes down to a decision by the customer of what size pump rating they would like to have.
One other consideration is future testing of the pump. As years of use take some toll on the pump it may be more difficult to obtain certain flow ratings from draft. If a customer purchases a 2000 gpm pump and 10 years later it cannot meet the 2000 gpm rating due to wear, it is considered a pump test failure, however that same pump might easily meet the 1750 rating after years of service. Therefore some customers will choose to only rate the pump at 1750 to assure future test success.

Where this discussion can become complicated is when you get into a discussion of what the pump can actually “flow,” or how it performs in different situations. This is the key point of this document, the actual pump performance.
Most firefighters refer to the pump size based on what they believe the maximum flow is. While there are several pumps on the market today that can achieve flows over 2000 gpm, the industry standard for the “big pump” has traditionally been the 2000 gpm rating. So many customers will specify that they want the full capacity 2000 gpm rating.

What most firefighters fail to realize is that the 2000 gpm rating is based on the pump rating from draft, which we covered in the pump rating section previously in this document. The key point here is that a 2000 gpm pump, operating from a pressurized water source (hydrant feed or feed from another apparatus) will flow well over 2000 gpm.
Aerial apparatus are a good example of where pumps are traditionally oversized based on this misunderstanding. A large majority of aerials don’t even carry suction hose, so they never expect to operate from a draft condition. But if a platform is equipped with a 2000 gpm waterway, then the customer feels the need to put a 2000 gpm “rated” pump on the apparatus. In reality, a much smaller “rated” pump will more than adequately flow the 2000 gpm waterway because the rig is being supplied from a pressurized source.

In the case of most pumps the reality is that whatever amount of water you can get into the pump, you can get out of the pump. Take for example a 1500 gpm pedestal style pump, the limitation that rated this pump at 1500 gpm has nothing to do with its ability to flow water out of the discharges, it’s the limitation of how much water the pump can get in from draft. The case study shown later in this document will help explain the flow capabilities of the pump.

In today’s economy cost is one of the first considerations in all decisions. The cost comparison cannot be just between the pedestal pump cost and the full body pump cost because the pedestal pump will require a manifold to be built by the OEM. So for cost comparison you would need to look at a completed pump module with each style pump.
On average, a full body pumping system is a few thousand dollars more than a pedestal style system.
The next consideration is pumphouse size. Of course there are several considerations with pumphouse size including but not limited to pump size, number of discharges, inlet options, foam systems, crosslays, storage in dunnage area, etc.

Comparing two configurations with the same number of suctions and discharges, the general savings in pumphouse width with the pedestal pump versus the full body pump is about half a foot.

There are many unrealistic things in life that can be proven with very detailed math equations and scientific theory. In the end, the proof is obtained through practical testing in the field to show true capabilities.
The following test was performed with a KME PRO Pumper, GSO 9755. This pumper has a UL certified pump rating of 1500 gpm. The details of the pumping system, apparatus powertrain, and test conditions are as follows:
  • Apparatus powertrain: Cummins ISX-12, 500 HP Engine
  • Transmission: Allison 4000 EVS
  • Pump: Waterous CXS Split shaft single stage pump
  • Pump Transmission: Waterous C20
  • Intake: 6” Inlet equipped with Waterous Monarch Valve and 5” Storz fitting
  • Test Discharge #1 (Deck Gun): 3” Elkhart valve with TFT Monsoon 1250 monitor
  • Test Discharge #2 (Front Bumper): 2-1/2” plumbing, 2-1/2” Elkhart valve with 2-1/2” discharge outlet
  • Test Discharge #3 (Rear Preconnect): 2-1/2” plumbing, 2-1/2” Elkhart valve with 1-1/2” discharge outlet
  • Test Discharge #4 (Rear Preconnect): 2-1/2” plumbing, 2-1/2” Elkhart valve with 1-1/2” discharge outlet
Test A – Draft
The first test was performed in a drafting situation. The 6” inlet was connected to 23’ of rigid suction hose with approximately 4’ of lift to the water surface. The deck gun was equipped with a 2” smooth bore tip that has a flow rating of 1246 gpm at 110 psi. Test discharge #3 was fitted with 50’ of 2” hose and a 2-1/2” smooth bore nozzle.
With this setup the apparatus was able to achieve a maximum flow of 1575 gpm from draft. The results  of this first test were right in line with what would be expected. The 1500 gpm pump was able to achieve its NFPA rating from draft and performed properly. Higher flows with the single suction setup were unable to be obtained as the apparatus had reached the capabilities of the pump to draft any more water in.

Test B - Hydrant
The second test was performed with the water source coming from a pressurized fire hydrant. The fire hydrant was fed from a 16” water main and delivered 110 psi of static pressure. The deck gun was again equipped with a 2” smooth bore tip with a flow rating of 1246 gpm at 110 psi. Test Discharge #1 was feeding 50’ of 3” hose to a TFT Blitzfire gun with a 1-1/2” smooth bore tip rated for 668 gpm at 100 psi. Test Discharge #2 was feeding 50’ of 1-3/4” hose to a smooth bore nozzle with a 15/16” tip. Test Discharge #3 was feeding 50’ of 1-3/4” hose to an automatic fog nozzle.
With each discharge at the appropriate pressures the apparatus was at approximately 1650 rpm, and the total flow output flow was 2,306 gpm. There was still 20 lbs of residual pressure coming into the apparatus, so another handline could easily be put into service and the apparatus could exceed 2500 gpm of total flow.
These test results are the key to this document, the 1500 gpm pump, was able to flow 2,306 gpm with just one pressurized source coming into the apparatus, without really pushing the pumping system anywhere near its limits.
Tying this back into the original discussion, a platform aerial device with dual guns and a 2000 gpm rated waterway could easily flow full capacity through the 1500 gpm rated pedestal style pump.

The decision on what size pump to put on an apparatus should not just be based on an arbitrary number. The department should consider their standard operating practices, primarily whether the apparatus will be required to flow from draft on a regular basis, or if the feed will typically come from a pressurized source to the apparatus. If the typical operation is from a pressurized source, then the “rating” of the pump should not be the primary concern, instead the number and size of discharge should be the focus.
The theory that what you can get into the pump, you can get out of the pump, is proven to be accurate in most situations. In most cases, both money and space can be saved by utilizing a more compact pump with a lower rating from draft, while still obtaining high flow on the fireground.
Posted: 6/29/2016 3:35:22 PM
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