The following is based on a true story about a physical plant stationary engineer who was seriously scalded and died from his injuries from hot boiler effluent overflowing from an undersized drain installed in a renovation project. The names of the people and places have been omitted; however, the story should be a learning experience for plumbing design professionals, mechanical engineers, contractors and plant operators.

The boiler plant was manned 24 hours a day, seven days a week, with one stationary engineer for each of three shifts to ensure constant oversight and operation of the boilers, chillers and other mechanical systems associated with the physical plant and its utility systems. According to police reports, the decedent worked the third shift. 

Before the end of his shift, a high-water alarm went off for a large, high-pressure steam boiler operating at a steam pressure of 150 pounds/square inch (psi) for distribution to many buildings. This boiler, which was a couple of years old, had a maximum allowable working pressure of 250 pounds/square inch gauge (psig), a minimum operating pressure of 100 psig and a maximum design steam capacity of 50,000 pounds/hour. 

The decedent went over to the boiler to lower the water level by following the procedure for a blowdown. He opened the blowdown valves located at the back of the boiler, and then he walked to the front of it to watch the sight-glass serving as a water-level monitoring gauge. 

While standing in front of the boiler, he observed the hot boiler blowdown effluent overflowing violently from the hub drain that received the discharge from the blowdown tank. He returned to the back of the boiler to close the blowdown valves and stop the overflow. 

At some point, the decedent slipped and fell on the scalding hot and slippery boiler effluent, which also burned his feet through his shoes. A co-worker, whom the decedent called after he fell, found the blowdown valves closed. The fall on the wet, slippery floor was enough to cause serious scald burn injuries through clothing. The unfortunate stationary engineer went through a lengthy hospital stay and endured several amputations of limbs before succumbing to his injuries. 

Litigation ensued, and the question became: Why was the hot boiler blowdown effluent overflowing violently from the hub drain that received the discharge from the blowdown tank? Everyone involved in the design and construction argued that a blockage was likely further down the drain piping in the branch drain line. 

However, this argument was a hopeful deflection of blame because a properly designed (sized) and maintained sanitary drainage system receiving hot boiler effluent should not become blocked — and there was no blockage. In fact, a video captured an earlier overflow of the hub drain during a blowdown of an adjacent boiler that also discharged into the same tank (at separate times); this occurred while the renovation was still ongoing. 

The video clearly revealed no blockage existed as a downstream drain (downstream from the hub drain that received the discharge from the blowdown tank) was not backing up or discharging water. 

The overflow occurred because the sanitary drainage system was undersized. Because this was a renovation, pre-existing sanitary drainage piping was in the building; the main drain line that led to the public sewer system outside the building was not replaced. Instead, new underground branch piping (the same size as the pre-existing main line) was installed. 

The defending parties seemed perplexed as to why the sanitary drainage system seemed to have been sufficiently sized before the renovation but not after the renovation. This is simple to answer: the old boiler discharge system relied on larger equipment and a different layout and processes than the new system. It incorporated a blowdown tank designed to receive and hold the discharge from only one boiler at a time.

10 Boiler Effluent Tips

Immediate notes for plumbing design professionals and mechanical engineers:

1. Boiler effluent is very slippery due to minerals, sludge and boiler water treatment chemical additives in the water (corrosion inhibitors and other chemicals).

2. All boiler effluent will be slightly under 212 F when released to atmospheric pressure through the opening of the blowdown valves for release through piping into a blowdown tank. It can even flash into steam, which escapes through a blowdown tank vent, then through the roof. 

3. Direct connections between the boiler system and sanitary drainage system are prohibited. Boiler effluent leaves the blowdown tank through an overflow dip-tube and enters the sanitary drainage system indirectly as the two systems (boiler blowdown system and sanitary drainage system) cannot be directly connected. 

Per the plumbing code, there must be an air gap or air break between the end of the discharge piping of the blowdown tank and the rim of the horizontal standpipe or hub drain (part of the sanitary drain) receiving the effluent and cooling water for transport through the sanitary drainage system. 

Sanitary drainage systems are not rated for pressure applications. The steam boiler system — including the steam boilers, the blowdown tank and associated piping — is a pressurized system. Pressure will disrupt the sanitary drainage system. 

If a steam boiler system were directly connected to the sanitary drainage system, then “live” steam could be discharged into the drainage system, blowing the water out of P-traps throughout the building and introducing live flash steam from fixtures into occupied spaces where a sanitary drainage system is interconnected throughout the building. 

This is one of the reasons why there must be an air gap or air break between the indirect waste pipe of the blowdown tank and the sanitary drainage system; another reason includes protection against backflow. For example, the 2009 International Plumbing Code (IPC) includes the following sections:

“701.7 Connections. Direct connection of a steam exhaust, blowoff or drip pipe shall not be made with the building drainage system. Wastewater, when discharged into the building drainage system, shall be at a temperature not higher than 140 F (60 C). When higher temperatures exist, approved cooling methods shall be provided.” 

“801.2. Protection. All devices, appurtenance, appliances, apparatus intended to serve some special function, such as sterilization, distillation, processing, cooling, or storage of ice or foods, and that discharge to the drainage system shall be provided with protection against backflow, flooding, fouling, contamination and stoppage of the drain.”

“802.1.5 Nonpotable Clear-Water Waste. Where devices and equipment such as process tanks, filters, drips and boilers discharge nonpotable water to the building drainage system, the discharge shall be through an indirect waste pipe by means of an air break or an air gap.

“802.2 Installation. All indirect waste piping shall discharge through an air gap or air break into a waste receptor or standpipe. Waste receptors and standpipes shall be trapped and vented and shall connect to the building drainage system. All indirect waste piping that exceeds 2 feet (610 mm) in developed length measured horizontally, or 4 feet (1219 mm) in total developed length, shall be trapped. 

“802.2.1 Air gap. The air gap between the indirect waste pipe and the flood level rim of the waste receptor shall be a minimum of twice the effective opening of the indirect waste pipe.”

4. Cooling of the effluent is required. Per the plumbing code, boiler effluent must be cooled down to no higher than 140 F (60 C) before entering the sanitary drainage system. For this reason, boiler effluent is piped into a blowdown tank to cool before discharging into the sanitary drainage system. The blowdown tank design allows the boiler effluent from a prior blowdown to remain in the blowdown tank until it is pushed up out of the blowdown tank by a subsequent blowdown. 

This is intended to allow boiler effluent to cool down a little (from 212 F to about 195 F). An aftercooler senses the discharge temperature and opens a cooling water valve if the effluent is higher than 140 F (60 C). 

5. According to the IPC, the maximum temperature of waste discharged into the sanitary drainage system shall be at a temperature no higher than 140 F. Cooling water may be added to the effluent in the tank. As the effluent leaves the tank, additional cooling water may also need to be added to the effluent before it enters the sanitary drainage system. 

The IPC includes the following sections:

“701.5 Damage to drainage system or public sewer. Wastes detrimental to the public sewer system or to the functioning of the sewage treatment plant shall be treated and disposed of in accordance with Section 1003 as directed by the code official.”

“701.7 Connections. Direct connection of a steam exhaust, blowoff or drip pipe shall not be made with the building drainage system. Wastewater, when discharged into the building drainage system, shall be at a temperature not higher than 140 F (60 C). When higher temperatures exist, approved cooling methods shall be provided.” 

“803.1 Wastewater temperature. Steam pipes shall not connect to any part of a drainage or plumbing system and water above 140 F (60 C) shall not be discharged into any part of a drainage system. Such pipes shall discharge into an indirect waste receptor connected to the drainage system.”

6. The size of the standpipe does not determine drain capacity. The size of the P-trap and the size/slope of the horizontal drain piping determine the flow rate of the sanitary drain, not the size of the standpipe or hub. The maximum drainage flow of the sanitary drainage piping is restricted by the maximum flow rate through the P-trap and horizontal sanitary drain. 

The opening size of a standpipe does not determine the drain size or capacity. Rather, the P-trap size and horizontal drain size are what determine the drain’s fixture unit capacity. The IPC includes the following sections with respect to the sizing and installation of standpipes:

“709.4 Values for indirect waste receptor. The drainage fixture unit load of an indirect waste receptor receiving the discharge of indirectly connected fixtures shall be the sum of the drainage fixture unit values of the fixtures that discharge to the receptor, but not less than the drainage fixture unit value given for the indirect waste receptor in Table 709.1 or 709.2.”

“802.3.1 Size of receptors. A waste receptor shall be sized for the maximum discharge of all indirect waste pipes served by the receptor. Receptors shall be installed to prevent splashing or flooding.

“802.3.2 Open hub waste receptors. Waste receptors shall be permitted in the form of a hub or pipe extending not less than 1 inch (25 .4 mm) above a water-impervious floor and are not required to include a strainer.

“802.4 Standpipes. Standpipes shall be individually trapped. Standpipes shall extend a minimum of 18 inches (457 mm) and a maximum of 42 inches (1066 mm) above the trap weir. Access shall be provided to all standpipes and drains for rodding.”

7. The size of the standpipe and P-trap should be sized to handle the effluent and cooling water and not exceed a half-full drain flow:

“704.2. Change in Size. The size of drainage piping shall not be reduced in size in the direction of flow. A 4-inch by 3-inch (102 mm by 76 mm) water closet connection shall not be considered as a reduction in size.”

“706.2. Obstructions. The fittings shall not have ledges, shoulders or reductions capable of retarding or obstructing flow in the piping.” 

8. Sanitary drain pipes must be sloped properly and sized for no more than half-full flow in order to not flood the pipe and cause venting issues and siphoning of traps or blowout of traps. Drainage pipes should be run with a uniform slope at the minimum slope for each pipe diameter identified in the IPC, Table 704.1. 

“704.1 Slope of horizontal drainage piping. Horizontal drainage piping shall be installed in uniform alignment at uniform slopes. The minimum slope of a horizontal drainage pipe shall be in accordance with Table 704.1.”

9. Plan drawings (plumbing and mechanical drawings) should give design-related information for the blowdown piping and accessories. This should be from the outlet of the boiler blowdown valves to and through the blowdown tank, the indirect waste discharge from the blowdown tank, the aftercooler, the air gap or air break to a hub drain, and to the building drainage system. 

System pressures and flow requirements for mud drum blowdown, continuous blowdown and cooling water should be provided, as well as flow capacity in gallons/minute (gpm) and pipe sizes, and calculations showing conversion of gpm to drainage fixture unit values for pipe sizing. 

10. Tying into existing building drains. To tie into an existing sanitary drain, the new and the existing load must be calculated in drainage fixture units to determine the required size of the drain pipe. The flow rate of the load (gpm) is converted into drainage fixture units using the “values for continuous flow” section in the IPC; this determines the size of drain pipe required for the new load. 

After this calculation, a survey of the existing drawings should be performed to determine whether there is an existing building drain nearby that is the same or larger size than the size of the drain pipe required for the new load. If there is no existing building drain nearby of the same or larger size than the size of the drain pipe required for the new load, then installing the new drain pipe into the existing sanitary drain should not be done because it violates the IPC. 

The IPC includes the following section:

“703.4 Existing building sewers and drains. Existing building sewers and drains shall connect with new building sewer and drainage systems only where found by examination and test to conform to the new system in quality of material. The code official shall notify the owner to make the changes necessary to conform to this code.”

In next month’s issue, part 2 of this column will delve into how to size a sanitary drainage system.