How long is it safe to run a rotary pump against a blind filter/pressure gauge? The pressure valve in the pump will be venting flow back to the inlet, but is it safe to do this for a longish period of time? (Say 30 seconds to 1 minute) I'm trying to test out an idea on the bench with a spare pump, and I just want to monitor pressure pumping against a gauge.

Well, from what I understand, backflushing presents no more of a danger, then pulling ristretto's.

Vibe Pumps have a duty cycle, the ubiquitous Ulka52w, IIRC, is 60sec on/90sec off. AFAIK, Rotary pumps don't have a duty cycle, and simply like to have water flowing through them. Since rotary pumps are used in many, many other applications then espresso machines(where they run for longer then 30seconds at a time), I think it safe to say you don't have to play by the same rules as a vibe pump.

I suppose, as long as your safety release is working, you could "backflush" for as long as you'd like. However, until someone more versed in this than I confirms it, Ill stick to "one minute should be fine".

I fully agree with the previous responses.

The only thing I will add is that this recirculating flow may creates an increase in temperature in the fluid because of shear forces at the relief/by-pass valve. For a short time this is of no concern but over a long time it could become an issue. I have no data on how the temperature would increase nor do I know when it could become detrimental but it might be worth keeping an eye on it.

Rotary pump ON for 40+ MINUTES, now what?

Would repeat that same mistake willingly? How often would it still be OK?

I think the OP is preparing a bench experiment where he will be doing it purposefully.

I am writting this to ensure that it is clear that once does not make a rule...

JmanEspresso wrote:Well, from what I understand, backflushing presents no more of a danger, then pulling ristrettos.

HB wrote:Given that a rotary pump's capacity is much, much higher than espresso brewing flowrates, the difference between pulling a ristretto and pulling against a blind basket is negligible, at least from viewpoint of the rotary bypass valve.

Why the choice of ristretto? If you have a 50 liter/hour+ (and mind you, it's likely you have a 70-150 l/h model if you have a stock pump from an espresso machine manfacturer) rotary model there is excess flow beyond what you get at the group when you flush without a portafilter at all, that is, the water debit of the machine. The flow restrictor in most espresso machines restricts the flow of the pump. Translation: the bypass on your rotary pump opens the moment your pump starts spinning, and is pretty much open the entire time it's on, regardless of whether you're pulling a ristretto, normale, lungo, or screen clearing flush. Yes, it will open wider the more you restrict the flow until it's open wide and bypassing 100% of its output with the backflush disk. These pumps can operate pretty much indefinitely in this fashion, provided that the motor driving the pump is rated for indefinite operation (mainly, the motor would need adequate cooling).

So, Max, if you ever have another 40 minute backflush adventure extravaganza, I would suggest checking the motor temperature (by cautious touch). Turning the machine off immediately and letting it cool off is probably the best thing you could have done.

If there's anything I'd be concerned with, it's the 3-way solenoid coil. I've noticed they get darn hot when they're on for awhile, hotter than they get just from being bolted to the grouphead. Does anyone know if those have a usage rating that is less than 100% on?

Anyway, Nick, if you just hook up your pump to an espresso machine and unplug the solenoid from a group, then activate the brew switch for that group, you can operate the pump 'blind' against the gauge without worrying about burning out the solenoid coil—not that anything bad would happen to it in ~1 minute in any case. If you're thinking 5 minutes, I'd think about unplugging the coil to be safe. But to be clear, there's nothing in the brew path of a commercial espresso machine that can't take 130-150 psi for an indefinite period, and you can touch the motor periodically to make sure it's not overheating.

However, bear in mind that if you do this with a hot machine, you'll get really crazy pressure readings due to thermal expansion. On most machines there's a check valve between the pump and machine, and when you pump cool water into the system at 130 psi and it has nowhere to go, and then the cold water heats up in the heat exchanger (whether you have a HX machine or a double boiler with an HX tube feeding the brew boiler), you will see the pressure start to rise up till it opens the OPV and levels off at the OPV setting. So unplug your heating elements and cool off the boilers if you want accurate pressure readings. Or loosen your OPV till it's set lower than the pump, though it will drip a lot if you do this.

Whale wrote:Would repeat that same mistake willingly? How often would it still be OK?

I think the OP is preparing a bench experiment where he will be doing it purposefully.

I am writting this to ensure that it is clear that once does not make a rule...

shadowfax wrote:So, Max, if you ever have another 40 minute backflush adventure extravaganza.....

Definitely a mistake, definitely will not repeat purposefully. Just thought the OP would find that thread interesting.

shadowfax wrote:If there's anything I'd be concerned with, it's the 3-way solenoid coil. I've noticed they get darn hot when they're on for awhile, hotter than they get just from being bolted to the grouphead. Does anyone know if those have a usage rating that is less than 100% on?

I went and looked up typical espresso machine solenoid coils from Parker, e.g. this one (Z series coil, p/n 304012), and they seem to draw 9-16W of power. So they probably ought to be OK for continuous duty, but it's hard to tell from the information I can find.

The RPM motor(?) in the Vetrano had/has a thermal cutout SIMILAR to that installed in a lot of vibration pump machines, i.e. it automatically resets itself. Really nice responses in this thread.

There is little to no advantage to leaving it on in this situation for longer than around 12 seconds.

NickA wrote:I'm trying to test out an idea on the bench with a spare pump, and I just want to monitor pressure pumping against a gauge.

Presumably the OP has some advantage in mind beyond backflushing. I got the impression he has technical mischief in mind.

coffee.me wrote:Definitely a mistake, definitely will not repeat purposefully. Just thought the OP would find that thread interesting.

Didn't want to imply you would but since the quoted thread finished in a no-fault-found with a cool Joe face...

shadowfax wrote:Presumably the OP has some advantage in mind beyond backflushing. I got the impression he has technical mischief in mind.

Well spotted Nicholas ...