Carbamate Plunger Pump packing 24 months lives fallen to 3 months & associated problems eliminated.
|Article Type:||Root Cause Analysis (RCA)|
|Equipment Type:||Miscellaneous Rotating Machine Problems|
|Author:||S. Raghava Chari|
Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.
Carbamate Plunger Pump Packing Problems
Carbamate pump plunger packing initially leaked after 24 to 26 months of service; now after 5‑years at 2 to 3 months intervals; the crew took it a way of life and lives with the problem for four more years.
Thanks to the standby pump, production loss is insignificant; however, the maintenance efforts and expenses have gone up exponentially. Hence, the author the new maintenance manager Root Cause Analyzed thus:
He learnt the following from the packing change crew leader:
- Crew changes the packing the way the vendor expert Herr Mueller taught as under
- Following Mueller advice, the workshop judges the blemishes extent, discard plungers likely to be less than 124 dia after blemish removal grinding, under size grinds the original 125 mm f7 plungers till blemishes free, super-finish them to 0.4 µm smooth and pass for superfinishing.
- Machinists super-finish the plungers using lathe bed mountable Mueller suggested German super-finish attachment, super-finishing stones and cutting oil
- A die and plunger set for packing ring compression came with the machine. Field crew compress inserted into the die each packing ring with 13000 kg load using the shops hydraulic press.
- Field Crew insert the plunger in the bought out spare barrel, insert the compressed packing rings and hand tighten the packing nut. This way they repack the other two barrels to ready rotable barrel assemblies.
- Rotating machine crew replace the readied barrels in place of the leaky barrels
- Operators condensate pressure the barrels to 220 bars using the air operated pump
- Mechanics tighten the gland nut enough to stop leaks
- The readied pump goes into service when the working pump barrels develop leaks
The author felt something wrong with this procedure but could not pinpoint then.
Constant thinking flashes the Root Cause to the author
The 13000 Kg compressed packing ID is 125 mm suitable for fresh 125 mm dia plunger. Hence, the clearance between random dia plunger and such packing clearance could be even 1 mm instead of zero right at the start.
The plunger bottoms on the packing ID leaving a 1 mm gap at the top. Plunger supporting guide bushes (GB) wear worsens the situation. The stuffing box gland nut tightening is inadequate to make up for the large clearance.
Thus, a new packing set starts with very unfavorable conditions to resist leaks. Hence, it does leak too soon! Carbamate leaking via the packing freezes into very hard sharp-edged crystals.
They score the plunger heavily; hence, the more frequent carbamate pumps’ plunger and packing renewals than ammonia pumps.
The problem started 4 years after commissioning because all spare plungers had been used and reconditioned plungers only were available from then on.
Below given are two possible solutions:
- Use custom die-set to suit each plunger dia – very impractical indeed; hence discarded
- Stop plunger under sizing; chrome plate and build to dia 125.2 mm, finish grind and to 125 f7 mm dia and super finish – very practical; hence implemented
- Condensate lubricate the packing instead oil as now, because of the below given reasons
- The author finds carbamate dissolved in condensate soapy and of viscosity > 43 Centi-Stoke; i.e. as good lubricant as lube oil
- In addition, unlike oil, first few packing soaked carbamate dissolves in the sealing condensate, does not crystalize into glass hard sharp-edged crystals; hence would not score the plungers – the greatest advantage
- Saves oil costs too
- This is not possible NH3 pumps, as the very low carbamate impurity remains dissolved in liq NH3 and never crystallize. In addition, liq NH3 viscosity << 0.43 CS; hence, not a lubricant
- The pump vendor appreciated the feedback, approved it and includes in pumps he makes then on
Author RCA solution Benefits
The realized benefits are:
- Very small packing leaks after 30 months, i.e. lot longer than Mueller’s 24 months
- The carbonate remains dissolved in the condensate, does not crystalize into sharp edged crystals; hence, does not score the plunges
- No plunger under-sizing and sealing the packing with condensate instead of oil offer the following benefits:
- No under-sizing; hence no discards; hence no new plungers purchase
- Plungers size is always constant; hence, the 125 mm bore dia compressed packing serves all plungers with zero start clearance the essential most requisite for long packing life
- Hence packing last 125-150% more than Mullers time
- Far fewer plunger reconditioning than frees the machine shop for other jobs and avoid paying outside vendors
- Condensate sealing eliminates oil costs
- The seen leaks become unacceptable after a month only; hence, rush packing jobs involving enormous crew overtimes gone. Comfortably scheduled crew change the packing avoids stress to personnel.
- Far fewer pump changeovers, associated total plant outage risks gone
- passing pump isolation valves prevented pump changeovers and consequent total plant shutdowns gone
- Supervisors, freed from rush after hours, holidays packing tasks and consequent missed family functions thank the author
- Electroplating vendors thank the author for the new business, but the following are sore:
- Replacement plungers’ sales lost pump vendor
- 90% sales lost Packing vendors
- Enormous overtime payments lost rotating machine crew
Too low-efficiency Torque Converter elimination attempt
The author considered eliminating the too low efficiency around 30% only torque converter, by using a power supply frequency converter aka variable speed drive (see Electrical Problem 20 for a description on VSD). Unfortunately, the 80s infant VSD technology offered 400 V VSDs only.
A US vendor offered converting motor 3.3 KV 50 Hz power into 660 V 50 Hz power, feeding it to the VSD and converting the VSD variable voltage, variable frequency output into higher variable voltage at variable frequencies to vary the motor speed.
Because of the high costs and too much modification involved, the author dropped the idea without much thinking.
Little more thinking would have shown by eliminating the torque converter instead of 450 200 KW LT motors suffice. The power savings would have paid back the modification costs in 2-3 months. The author and all missing this is sad indeed.
A mid-eighties Swiss vendor-supplied 6.6 KV VSD for a Saudi Arabian cement kiln drive surprised him. The user was immensely happy with the negligible maintenance requiring and high-efficiency drive.
Author: S. Raghava Chari
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