Shunt Trip - Not Working Correctly

[Sidewinder]

Are you sure the lube pump is not a safety device.

If the lube pump failed, what would happen to the compressor.

If the worst happened to the compressor, could any collateral damage or injury to persons or other property occur?

If so then the lube pump could be considered a SRPCS.

 

[Technician]

I have come to this late...apologies.

Just to add to Sidewinder's points, IIRC the

safety features in refrigeration are a low lube

oil which is a differential pressure switch linked to

the ammonia suction. This may be arranged to

self reset, it needs to so that if suction pressure rises

the compressor can restart because the expansion valve

in the high side determines the flow rate of the refrigerant.

Equally, the low oil pressure should cause the compressor

to stop.

There must be a high pressure cut-out with a MANUAL

reset, and this can be set above another high pressure

cut out that can be self resetting. The inclusion of this

may depend upon the designer.

That HP manual reset cut-out is a must.

Without my notes I cannot offer much more but If I am

repeating any other posts, apologies.

HTH.

Just an afterthought; The compressor will have a bursting

disc somewhere so that closure of the discharge valve will

cause the disc to fail which will relieve the refrigerant to

the suction. The last thing you want is an event where the

HP cut out fails and the bursting disc operates because it

means opening up the compressor with all that this entails.

 

[Technician]

I knew of a case where a sand moulding machine had

an E-stop (Emergency stop) system attached to the

gates to the moulding chamber. The gates had proximity

switches on them and the original design (as far as I could

surmise) was that these were wired in series to operate the

E-stop. Trouble was, someone tinkered with them and

(as far as I could surmise) wired them in parallel. This

nearly resulted in the operator losing an arm below the shoulder.

 

[Technician]

Canoeboy...just a reminder...you forgot Pressure Systems

Regulations...thought I would get that one in before

Sidewinder...

:Salute

Cannot imagine why LOLER should be in there; unless you

have to crane out the scrap after the event.

But funnily enough only four items in the list above feature

in a NEBOSH course.

 

[Sidewinder]

Canoe, you missed the Low Voltage Directive, CDM, and EN954, which is sort of still current... (watch this space job), there are probably others but I can't remember now and got to go back to work!

Technician, which NEBOSH course?

I can't remember which regs were covered on my NEBOSH GC as I knew about most of them before I started the course!!!

LOLER is a safety related piece of legislation, it may not be electrically linked but it is a piece of statute law related to industry etc.

 

[Technician]

Sidewinder, I asked about a lot of the stuff on my NEBOSH

and I was told that LOLER, Pressure Systems and a couple

of others were "too specialised" and thus were not handled

in depth but only "mentioned in passing". I had no questions

on either of these.

You are right, there is nothing electrical in LOLER but I do

remember a fascinating conversation about the release time

for a DC brake with a crane inspector. Most of the brakes he

had seen had AC coils and when this DC one turned up, he

was at a loss to understand why there was a long delay

between energising and closure and vice versa.

 

[Sidewinder]

We covered a lot of stuff like this on my NEBOSH General Cert as the lead tutor was from an engineering background, and the 2nd tutor was a chemist / Occ Health type bod.

I was even asked to deliver the electrical section of the course!!!

I almost did do.

I had a cracker of a part 2 question in the exam covering the RA, MS etc. for working on an electric motor connected to a hot chemical pump!

Reckon I got near 100% on that one.

I could see the rest of the class looking around at me when they read that Q.

As far as slow d.c. brakes go, there is no reason for this to be so from a fundamental level.

The company I worked for always used d.c. brakes on the permag servo's as they could get them to actuate quicker than an a.c. design.

Critical when you are positioning machine tool axes.

It could be that there were design, applications or other reasons for the slow energising of the brakes, but, they can be faster than a.c.

 

[Technician]

Sidewinder it was an old crane, probably 10 ton. IIRC

correctly the crane was installed post 1940 to 1945.

Again, IIRC the motor was AC but the older DC brake

had been retained.

I agree, modern DC techniques are a lot faster but in

this case he had been examining it for years and always

wondered WHY the brake took so long to open when

the hoist or lower button was pushed.

 

[Technician]

I have saved the article to read later; tnks.

The brake type was the old two-shoe brake

with a drum, the shoes closing on the outer

surface of the drum. Brake operation (as usual)

was to energise the coil, compress the spring

and open the caliper hinged from the bottom.

Old hat but it worked. Same as a lift brake in

many designs IIRC.