A fraction too much friction

Friction is the resistance a surface meets in moving over another. We rely on it when we apply the brake in a car, or hold a hot cup of coffee.

Friction is an important consideration in plumbing design. For large projects, the plumbing hydraulic consultant needs to allow for frictional losses in the design of services such as water, gas, heating and ventilation.

For small projects, plumbers also need to understand the effects of friction. Proper sizing of pipes and ducts, secure pipe clipping and protection against abrasion all contribute to the performance and durability of plumbing.

Here we look at some plumbing problems where the consumer was rubbed up the wrong way…


Heating ducts drive you around the bend

Recently, we inspected three new 2 storey townhouses in a row, south of Melbourne, each with a ducted central heating unit installed in the first floor ceiling. Unfortunately, there was very little air from some outlets, particularly at ground floor ceilings. One heating unit was shutting down regularly via the safety high temperature cut-out switch.

Inspection of the controls and gas supply revealed that they were adequate. The central heating units had sufficient capacity to heat the townhouses. The units were rated at 20 kW and given the size of the internal areas, could deliver 45 W/m³, and this would have been satisfactory for these particular buildings.

We found that all three systems had poor air distribution. The air velocity at some ground floor outlets was 0.4 to 0.5 m/s at around 48 deg C. The supply air temperatures should have been in the range of 55 to 62 Deg C, and outlet neck velocities should have been in the range of 1.5 to 2.5 m/s.

The gas-heating units were designed to deliver around 620 litres per second of supply air for 11 outlets. Thus the design supply air quantity per outlet was 56 L/s. The measured quantities per outlet averaged only 15 L/s. We also measured the return airflows to verify the air quantities, and ensure that the supply air was not disappearing somewhere.

Generally, the flexible ducting in the ceiling spaces had excessive lengths and abrupt bends, and this caused poor airflows at affected outlets. Poor air distribution (ductwork, grills etc) can also cause poor airflows at air outlets. Poor airflows can cause high delivery temperatures, and in fact caused one gas heater to shut down regularly via the safety high temperature cut-out switch.

While the duct sizes appeared adequate, we found that the air supply outlets were slightly undersized. Each air outlet had a 150 mm neck diameter. Outlets of this size should be limited to airflows of around 40 litres per second.

It is vital for plumbers to fully consider the airflow friction losses and air velocity in flexible heating ducts especially to the furthermost outlets through proper duct sizing and reducing excessive lengths and abrupt bends.

Equally important is designing a balanced duct configuration so that outlets close to the heater don’t have excessive air velocities. High air velocity through ducts and outlets causes air friction noise, and this is undesirable, particularly in bedrooms.

Pipe abrasion

Excessive friction, particularly on water pipes, can cause abrasion resulting in burst pipes and property damage.

Case 1

Pipe hammer can cause friction and abrasion when a valve closes rapidly and there is pipe movement.

Early this year, we investigated the cause of water damage to a hospital in Melbourne. The water damage included wet area floor and plaster wall; and adjacent office timber floor and carpet.

A flexible stainless steel braided hose water connection to a basin had burst. Each time the hot water flick mix tap was operated, the stainless steel braided hose would move and rub against an edge of the porcelain basin causing the stainless steel braiding to fray and hose to burst. This flexible connection had been installed for 7 years.

Flexible stainless steel braided hose connectors are approved for use in Victoria, but are never clipped. Australian Standard AS/NZS 3500.4: 2003, Section 4.5 states that,” Water services installed above ground shall be retained in position by brackets, clips or hangers” and, “Brackets, clips and hangers shall be clamped securely to prevent movement, unless designed to allow for thermal movement”.

It’s unlikely that these flexible connectors will ever be clipped properly, but highlighting the potential problem might prevent a reoccurrence.

Case 2

Excessive friction can cause pipe stress fracture when there is lack of provision for pipe lagging / wrapping.

Late in 2005, we were called to inspect a fracture in a cold water supply pipe. The pipe passed through a brick wall and was connected to a hot water unit. With minor settlement of the hot water unit, the pipe was pulled down on the brick wall causing a pipe fracture.

The pipe had not been wrapped with lagging through the brickwork. In this case, if the piping had been lagged through the brickwork, the piping would have been free to move with low friction and the fracture would not have occurred. As in the previous case, the plumbing was only 7 years old.

AS/NZS 3500.1: 2003 requires that pipes through masonry or concrete be lagged or wrapped with an impermeable flexible material. Unfortunately, there are few inspections to pick-up these defects in workmanship, and property owners are left with the defects and resultant water damage.

Summing up

Friction is a broad subject, and perhaps a little off the wall. Nevertheless, it is a relevant and important consideration in many facets of plumbing design and installation.

Peter Wenning is a Melbourne based plumbing consultant. He can be contacted at pwenning@bigpond.com