Developing & Implementing Maintenance Tactics
405.03
HANDOUT #1 – Operating Context for Weld Water Pumps
The automotive assembly plant produces 60 units per hour, and operates on 3 shifts per day, with
normal operation of 5 days per week. The workforce consists of 800 hourly unionized workers per
shift.
The assembly plant is divided into 3 major areas, in order of process operation: BIW (Body In
White), Paint, and Final. In BIW, metal assemblies and components received from suppliers are
fastened together precisely in tooling using resistance welding or through fasteners. In Paint, the
bare metal bodies are prepared for paint, primed, and the paint finish is applied. In Final, the
vehicle is finished through the addition of drive train, suspension, glass, interior, etc.
Parts are delivered to the plant from their suppliers on a scheduled basis (JIT – Just In Time), with
only about two hours of accumulated storage to allow for potential traffic and weather problems.
Similarly there is about two hours WIP (Work In Process) accumulation between BIW and Paint,
and between Paint and Final to allow the paint processes to be loaded and unloaded. As all
vehicles scheduled to be built are scheduled for delivery, any loss of production must be made up
through overtime at a cost of $5,000 per minute.
The welding in BIW is done using resistance welding. In resistance welding, the sheet metal
components and assemblies are clamped together in the tooling. A weld gun closes over the
weld point with copper contact points (weld caps) on both sides of the sheets to be welded. A low
voltage, high current passes between the contacts resulting in a high current density between the
metal sheets and fuses the two sheets together at the spot between the weld caps. The weld then
cools and the weld gun opens with the weld complete.
To reduce the heating of the weld caps and reduce their distortion, the caps are cooled by
circulating water behind them. The cooing water is also used to cool the weld transformers and
cool the SCR’s used by the weld controllers. Each of the 600 weld guns requires a total of about
3gpm of flow for the weld gun, weld controller and weld transformer. The cooling water extends
the life of the electrical equipment. The cooling water is however critical to the weld quality, and
should the weld quality be poor it will negatively impact the integrity of the vehicle causing all of
them to be repaired or scrapped. Inspection and repair time in the case of a weld water
overheating failure is estimated to be on average about 15 minutes per vehicle.
To reduce corrosion in the system and eliminate bacterial growth, the weld water is treated, but
the ph level remains neutral (neither acidic nor alkaline), and is not considered hazardous to
humans. Heat is removed from the closed loop circulating water system with plate type heat
exchangers and a storage tank provides additional system volume. Water from the utility can be
added to the system in the storage tank. The other side of the heat exchangers is connected to
an open loop system with two cooling towers. There are three 100hp pumps connected to
common headers on the suction and discharge side of the pumps to circulate water through the
cooling towers, with only two pumps required to be operating at a time.
On the closed loop system there are three pumps rated at 1,000gpm each. The three pumps are
connected to the storage tank through a common header on the suction side, and connect to a
common header to discharge side pumps through a press filter and into the heat exchanger. The
closed loop circulating system requires at least two pumps to be operating during production to
maintain flow above 1,800gpm. Flow through the heat exchanger is monitored using a local
Module 5 – Version 09.01
405.03.1
Maintenance Management Professional
MMP
1 of 2
Developing & Implementing Maintenance Tactics
405.03
HANDOUT #1 – Operating Context for Weld Water Pumps
reading flow meter. The water temperature needs to be maintained at below 25⁰ C, and is
monitored and alarmed remotely by a control room operator on a SCADA type (Supervisory
Control And Data Acquisition) system. On hot summer days if the temperature exceeds 25⁰ C,
the Operator is to monitor the temperature hourly to ensure it remains below 30⁰ C. The SCADA
system alarms when the circulating water temperature exceeds 25⁰ C, and the system is
shutdown when it exceeds 30⁰ C.
The three circulating pumps, storage tank and heat exchanger are located together in an area
providing secondary containment in case of a leak or spill, and can be drained to the waste
treatment plant.
The 1,000gpm circulating pumps are centrifugal pumps and are each coupled to a 75hp 1800rpm
TEFC foot mounted motor. A machine guard is mounted above the coupling to prevent accidental
contact with the rotating coupling. The pumps are connected to the headers via flexible couplings
to reduce stress and vibration. They have a manual valve on the suction and discharge side of
the pump to allow them to be isolated for servicing. An MCC (Motor Control Centre) in the area
provides power and controls for the motors. The motors are started and stopped manually by
production each day.
The brief (and incomplete) RCM analysis will address:
1. The 1,000gpm circulating pumps, both while operating in the duty mode, and in the standby
mode; and
2. If the system was installed without standby units.
Module 5 – Version 09.01
405.03.1
Maintenance Management Professional
MMP
2 of 2