Case Studies

Introduction

Water being scarce resource, its treatment, storage and distribution to public uninterruptedly is a challenge to most operators. Large WTP/WWTP use high capacity pumps for transfer of treated water. In most plants pumps are driven by electrical drive which draws huge current during start up. Reducing starting current and stress on conductor and drive poses challenge to most electrical engineers. Thanks to technology that helped to address this challenge and today most high capacity pumps are equipped with soft starters. Here is a simple case study on elimination of its failure.

Challenges

Soft starters minimize the starting current and ensure smooth ramp up by switching IGBT/ SCR thereby mechanical stress on motor and driven equipment is drastically reduced. 50MGD WTP in Dwaraka Delhi treats raw water and supplies potable water to nearby residents. Treated water pumps are driven by 110Kw,415V motors. Installed soft starters are Danfoss MCD500 internal bypass type(fig.1) thus motor starts with soft starter and once full speed is achieved load is seamlessly changed over to bypass contactor and motor runs on DOL (Direct on line) mode. Soft starters were frequently failing and warranting frequent repair and replacement of control and power cards. On an average INR 6lacs PA was spent to keep treated water pumps running on account of this alone. This has inflated OPEX and jolted complete economics.

Principle Cause

Detailed RCA (Root Cause Analysis) was done and concluded that soft starter internal components were failing due to temperature. While drive running surface Temp. of soft starter was measured with IR temp. gun and found high (Table 1). Therefore it is very evident that PCB and components would be certainly higher than OEM recommended temperature which is 50 deg.c. Temperature rise is due to heat dissipation from internal bypass contactor which is continuously ON while drive is running soon after soft starter goes to bypass mode. There was no quick means of removing this heat due to compact design. As a result, power card and control cards started heating and components started malfunctioning finally leading to soft starter failure.

Resolution

Now the challenge is to isolate heat sensitive components from heat dissipating components. Hence internal bypass soft starter has been retrofitted with external bypass soft starter which has its bypass contactor outside the soft starter (fig 1). Previous version was replaced with Schneider ATS48C25Q soft starter. Post modification significant reduction in temperature of about 16 deg C was evident (Table 1).

Figure 1. GA of soft starter with internal and external bypass

Conclusion

100% uptime of equipment in water treatment is critical for known reasons. This makes us constantly thrive for higher MTBF (Mean time between failure). Higher MTBF not only reduces OPEX but also improves brand in such sensitive public related business. Retrofitting of internal bypass soft starter with external bypass irrespective of rating at right time and consideration of this scheme initially in capital projects makes sense in such competitive industry. ROI is just less than a year!