System – Wet Fire Sprinkler

• Overview

Wet fire sprinkler systems always carry water, and therefore experience far less deterioration than so-called “dry” fire sprinkler systems.  Corrosion activity is fairly predictable, with the distribution and branch pipe easily inspected due to its generally open access at the ceiling level.  Fire standpipe systems, commonly installed in stairwells, produce even more favorable results given their less frequent testing, and of all fire protection systems, are the most trouble-free.

• Common Problems

Corrosion activity is typically low at stagnant piping systems having no flow of water.  This is because oxygen is expended during the corrosion process, which leads to lower and lower corrosion activity over time.  Were one to take a section of threaded pipe, fill it completely with water and cap it, a small amount of corrosion would take place before the oxygen level decreases and the corrosion process almost stops.  Open that same pipe section every month to change it out with new freshwater, and the entire corrosion process begins again.  The deterioration of a wet fire sprinkler system, therefore, is almost entirely dependent upon how much fresh water enters the system.

Our investigation of extremely old New York City warehouse buildings from the early 1920s have documented the fire sprinkler pipe in near new condition due to the fact that they were rarely tested and therefore contain decades old water having depleted levels of oxygen.  For one ultrasonic investigation of a downtown San Francisco office building which was knocked off its foundation and partially burned in the 1906 earthquake and firestorm, testing at its new threaded galvanized steel fire piping installed in 1907 identified it in near new condition – more than 105 years later.

In the photo at left, the removal of a 6 in. steel fire standpipe riser installed in 1928 showed barely a blemish of rust, and its wall thickness still 10% above standard specifications.  In order to move the riser 4 ft. away in order to satisfy a new tenant’s floor space demands, this pipe was replaced with undersized schedule 10 Korean ERW pipe.

While some of the oldest examples of fire sprinkler pipe had been constructed using extra heavy wrought iron or schedule 80 carbon steel pipe, most fire sprinkler systems preceding the 1980s were constructed using standard or schedule 40 pipe.  This specification has now changed to the predominant use of thinwall schedule 10 pipe and even ultra thinwall schedule 7 pipe in some examples.  Having approximately 50% lesser wall thickness for schedule 10 pipe, the impact is obvious for any wet fire sprinkler system having anything but low corrosion activity.

A further impact in terms of design relates to the now permitted installation of thinwall schedule 10 pipe for the threaded branch lines, where heavier schedule 40 pipe had been previously mandated.  Once again, such significantly less wall thickness compounded by a further 50% wall loss at the threads means that problems will occur under anything but the lowest corrosion conditions.

Often ignored is that once 1 in. thinwall schedule 10 pipe is threaded, its already low 0.109 in. beginning wall thickness minus a thread cut of 0.070 in. leaves only 0.039 in. of pipe wall remaining on the day it was installed – a dimension just slightly heavier than a standard credit card.  Add in the fact that even thinwall schedule 10 pipe is manufactured further undersized to often 10% less than its 0.109 in. specification, and the level of threat only increases.

Viewed only by its reduced initial cost and increased water flow permitted by a thinner pipe wall having a larger I.D. has greatly expanded its installation.  Yet when it fails prematurely, the first reaction is one of surprise.

• Major Threats

Although wet fire sprinkler systems and especially fire standpipes generally present significantly less concerns then other building piping systems, their failure have been increasing due to various conditions.  Top issues are the use of thinwall pipe in areas of higher fresh water flow, followed by air entrapment.  Undersized and generally lower quality steel and galvanized steel pipe adds further negative impact.

As for all piping systems, corrosion and failure problems are more frequently encountered at more recently installed fire systems; UT investigations of older systems often producing remarkably excellent results.

• • • • City water inlet

Recognizing the correlation between higher oxygen levels and corrosion activity typically results in an ultrasonic assessment of any wet fire sprinkler system showing highest deterioration at the very beginning city water inlet area.  Whether having its own separate water supply from the street or a take-off from the main domestic water system, this area of pipe always experiences the highest oxygen levels given water movement and migration from the active street main.  For some properties, they are now replacing their inlet piping for the 3rd or 4th time.

As water moves downstream through the pipe and into the building, the oxygen is again depleted as part of the corrosion process and is reduced – thereby resulting in lower corrosion rates downstream.  Given the generally stagnant nature of the distribution and branch lines, lowest corrosion activity is generally identified as one moves further into the building.

• • • • Air bound lines

A very common problem for large warehouses where the fire sprinkler distribution branch piping feeds upward from a main header is the presence of air pockets.  Building structures having arched or bowed roofs also have their branch lines installed parallel and close to those roofs; their uppermost piping now essentially providing compressed air storage.

When originally designed and installed, the volume of water calculated for the system also defines the volume of air at atmospheric conditions.  As the fire system is filled from the floor level, it feeds upward to displace the air into a smaller and smaller volume.  This relationship is defined by “Boyles’ Ideal Gas Law” and the formula (P * V) / T = (P * V) / T, where P equals pressure, V equals volume, and T equals temperature.  Knowing air temperatures at the floor and ceiling levels, the volume of the system, and its operating pressure then allows an easy calculation of how much air volume remains after filling.

For any large warehouse having a ceiling height of 30 ft. or more, significantly higher temperatures can exist at the level of the fire sprinkler piping.  This is especially true in Southern California, Arizona, Texas, and other such areas.

The result is that the air is compressed only to the degree predicted by this universal gas formula – resulting in an almost predictable air / water boundary at its upper lines.  Contrary to arguments that the air simply “disappears” somehow from the piping, it does not, and remains to provide a significant reservoir of oxygen to now accelerate the deterioration of any steel pipe in that localized area.  In effect, the wet fire sprinkler pipe at this air / water boundary now exists under the same greater threat as a dry fire sprinkler system also containing a mixture of air at the top and water along the bottom.  This is well illustrated at left at a section of roof level 2-1/2 in. distribution line which is airbound; the majority of the line filled with air.

This same condition can also exist where long horizontal runs are improperly installed to have high points, or need to travel above beams or air conditioning ductwork.  Where installed using thinwall schedule 10 or ultra thinwall schedule 7 pipe, the added condition of oxygen fueled higher corrosion activity will often lead to premature failure.  Resolution to the problem is to remove the air, which is difficult at most fire system designs have hundreds of air bound pipe sections per zone; each requiring an automatic bleed.

• • • • Expansion and renovation

Today’s frequent change of ownership, tenants, and use of building properties often requires a modification of the fire sprinkler system to meet new office designs, tenant demands, storage needs, etc.  This in turn results in potentially extensive renovations to the fire system requiring frequent draining and filling, which then leads to substantially higher corrosion rates.

Prior to any fire sprinkler system renovation, planning should be made to minimize the need for draining and refilling of the system since each event will replenish the system with fresh oxygenated water resulting in higher corrosion activity.

• • • • Thinwall pipe

As discussed, the fire industry has adopted thinwall schedule 10 and even ultra thinwall schedule 7 pipe for wet fire service.  Having ultrasonically inspected many such schedule 10 systems, we have been able to document very extended service life as long as corrosion activity is low, and where there are none of the higher threats present.  The installation of thinwall pipe, by definition however, always introduces the potential for a more advanced and unexpected failure compared to schedule 40 pipe.

This fact is especially true for any newer system installed today given the higher prevalence of lower quality foreign produced pipe, the manufacture of pipe closer to ASTM’s minimum wall thickness allowance, lower quality or defective ERW seams, and a higher frequency of renovations, testing, and draining.

• • • • Fire booster pumps

An extremely common location for pipe failure at any larger fire sprinkler system is at the supply pipe to the fire booster pump.  This condition is once again related to the introduction of fresh oxygenated water at this inlet area closest to the city water supply which is then further compounded by any loss at the pump itself.  All fire booster pumps have a packing seal which is designed to slowly leak water in order to provide cooling during its operation.  This loss of water is not just during testing or use, but 24 hours per day.  More advanced mechanical seals, common today to virtually all other HVAC and plumbing systems, which do not leak water, are considered unreliable for fire protection service.

While the volume of water released from the seals is typically minor, often not more than a small drip, it still results in the introduction of many gallons of new fresh water from the beginning of the system to the pump on a daily basis.  This in turn produces an accelerated deterioration of the inlet piping.  This movement of water is typically stopped by the check valve at the discharge of the booster pump.

In order to ensure a greater longevity of the supply pipe to any booster pump, we always recommend the installation of schedule 40 pipe exclusively, and to maintain the lowest possible leakage through the packing gland.  In the example at left, a 24/7 massive leak through the pump packing was addressed by placing a canvas rag over the seal and end bearing in order to contain the water spray from reaching all over the mechanical room floor.

After years in this condition, approximately 100 ft. of 8 in. inlet fire pipe was destroyed and required replacement.  An ultrasonic inspection prompted by multiple pinholes through the pump inlet line revealed all pipe downstream of the pump’s check valve in near new condition.

In another example, a metal cover over the leaking pump packing redirected its seal leak to a hole in the cup which released water onto the basin for the motor and pump, which was piped to a floor drain.  Problem solved, or maybe not.  After years of constant water loss, estimated to be near 1/2 GPM, the entire 8 in. inlet line comprising approximately 100 ft. to the pump was destroyed.

In addition to damage caused to the inlet piping, water collecting at the basin initiated a corrosion condition between the pump base and basin itself, the rust product expanding sufficiently in volume to misalign the pump to the motor shaft resulting in severe vibration and bearing damage.

• • • • Fire lines as a water supply

Adding a garden hose connection to a fire sprinkler riser or supply line to a sink or wash basin is not as uncommon as one would imagine.  Doing so causes the exact same higher corrosion activity discussed previously at fire booster pumps, but to a far greater degree given the higher water flow and greater length of pipe involved.

Although convenient in some examples that we have seen, every effort should be made to minimize the movement of fresh water through a fire sprinkler system in order to control higher corrosion activity.

• Testing Focus

Once recognizing that fresh oxygenated water introduced into a fire sprinkler system facilitates its deterioration, methods to minimize this wall loss become self-evident.  Any ultrasonic investigation should focus at the beginning of the system, and especially so where a fire booster pump is present.  Fire pipe running underground has the additional threat of external corrosion which is a general unknown.  With ultrasound difficult to perform unless excavations are made, monitoring water usage becomes the only diagnostic option.

Observation is always important, followed by the proper corrective action.  Areas of interest for any wet fire system investigation are:

• • • City water inlet piping
    • Main pipe serving a fire pump
    • Main riser
    • Any galvanized steel pipe
    • Jockey pump lines
    • Steel fire water storage tanks
    • Pressurized fire sprinkler tanks
    • Cold or sweating fire lines
    • Top or upper level pipe lacking air bleeds
    • Pipe installed below the ceiling
    • Fire pipe which has been insulated
    • Old riveted pressurized fire sprinkler tanks
    • Make-up water lines to any fire water storage tank

Noticing that the fire sprinkler pipe in a pump house is cold to the touch and at a substantially lower temperature than ambient is a sure sign of a downstream water leak.  Finding water droplets from condensation at any fire line means there is some loss of water downstream which is constantly introducing cold fresh water through the lines.  A parallel line of rusty water stains at the floor directly under the fire system also tells the story, as shown in the below center photograph.  Finding and correcting the leak immediately would be the appropriate response, yet it took repeated complaints from car owners before a leaking drain valve was replaced.

For one client, as shown above right, a different solution to the problem was to simply insulate the pipe in order to avoid a wet and slippery floor.  With constant fresh water flowing through thinwall schedule 10 steel pipe, a large volume of the system ultimately failed – its first indication being rust product leaking from the seams.

Corrview maintains a large photo gallery specifically related to wet fire sprinkler piping in addition to other corrosion related issues. To visit our gallery on wet fire sprinkler systems, please click here

© Copyright 2023 – William P. Duncan, CorrView International, LLC