Re: Roots Blower Steady Pressure Increase
Dear Pablonici,
Before getting into detail, there is some more information needed.
1)
You state: “P1 response was hardly constant with slight fluctuations”
I understand “hardly constant” as fluctuating and “slight fluctuations” as almost constant.
I assume that the blower pressure P1 fluctuated in phase with P2 from 0.6 bar to 0.65 bar.
2)
You did not give a value for P2 and whether this pressure P2 also changed in time.
3)
Have you checked the main collector pressure still being -130 mbar?
4)
Conveying rate is still the same?
Increasing the pressure settings of the blower has consequences for the power consumption (E- motor capable?) and outlet temperature.
Check the blower name plate.
The eductor is a device that converses pressure energy into velocity energy, whereby the pressure decreases below atmospheric, sucking material in and in a venturi, this kinetic energy is converted into pressure energy again (P2)
Subsequently, the pressure P2 is the conveying pressure for the pneumatic conveying system. The pressure P2 is reduced in the pipeline to the end pressure of -130 mbar.
My first guess would be that the venturi is worn out and not working properly any more.
However that should also affect the pneumatic conveying rate and -pressure.
All for now
Teus ■
Teus
Re: Roots Blower Steady Pressure Increase
Dear Teus,
Thanks for your answer,
1)
You state: “P1 response was hardly constant with slight fluctuations”
I understand “hardly constant” as fluctuating and “slight fluctuations” as almost constant.
Sorry for my english, I wanted to say "almost constant, hardly fluctuating". In fact, at the beginning, for one day we tried different dosing rates from 0 to 250 kg/h and we noticed that P2 decreased proportionally from 0.08 bar to 0.045 bar, but P1 fluctuated alternatively up and down (not in phase with P2) between 0.6 and 0.65 bar
2)
With time, P2 has increased together with P1, but I should check the records in order to provide more useful values.
3)
Yes, main colector pressure remains almost constant around -130 mbar
4)
We have been trying different rates, but normally we have kept them around 75 kg/h, and with this constant rate, likewise, we noticed that discharge pressure steadily increased.
About discharge blower temperature, at the beginning, it took it some minutes to stabilise around 100ºC; then this stabilising value incresased a little bit until current 110ºC.
I agree with your considerations that P2 is the pressure that we have for conveying the material, and together with the help of the -130 mbar, isn´t it?
When we made tests by means of disconnecting the pipe from the main colector we were removing the favourable effect of the -130 mbar and even then, material seemed to be being conveyed reasonably good.
If the eductor was worn out, should not we detect that sucking material process at discharge point would not be being done properly? Originally, instead of the Venturi, we had a rotary valve that made impossible that we could work because of the back flow (since our dosing rate is quite low and we had not a proper hopper plenty of material in top of the rotary device for sealing against back flow). If the Venturi was not working any more, I´m afraid that we should notice that material would not be discharging since pressure from the blower would try to get through that side.
By the other hand, if Venturi is worn out, should not have decreased P1 instead of have increased?
Our Roots blowers have a cooling fan that starts up together with the main blower. Another interesting data that we have noticed is that if we start up the blower without the cooling fan (doing it manually), blower discharge pressure increases too.
Thanks a lot again for your help and do not hesitate in asking for more data if you are interested in them.
Best regards ■
Re: Roots Blower Steady Pressure Increase
Dear Pablonici,
I understand that increasing the feeding rate from 0 to 250 kg/hr resulted in a decrease of P2 from 0.08 bar to 0.045 bar.
One would expect a higher conveying pressure with increasing feedrate.
That raises the question, where P2 is measured.
Not at the intake of the eductor and unlikely at the beginning of the conveying pipeline.
Leaves the location in the mixing chamber of the eductor, just before the venturi in which the pressure is built up again.
Another optional conclusion would be that the collector under pressure (-130 mbar # 0.13 bar) is completely responsible for the pneumatic conveying.
An eductor has 3 zones with pressures and velocities:
1)High inlet pressure from air supply (P1) with low velocity
2)Chamber with low pressure and high velocity (Probably P2)
3)After eductor (venturi) where the pressure equals conveying pressure and with conveying velocity.
Can you supply the following data for a capacity calculation?
Particle size
Particle density
Bulk density
Horizontal length
Vertical length
Number of bends
Pipe diameter(s)
Blower data from name plate.
Measured pressures at a measured rate.
From these data and the calculation, it might be possible to detect a possible cause. (I succeeded before in this way)
Also a description of the eductor would be helpful.
If the venture is worn out, the pressure build up for the conveying pipe is not sufficient anymore and a pressure rise of P1 could compensate that.
Have a nice day
Teus ■
Teus
Re: Roots Blower Steady Pressure Increase
Thanks for your help Teus; tomorrow I will try to provide most of the required data
Best regards ■
Re: Roots Blower Steady Pressure Increase
Pablonici
Good Day
It sounds like you are injecting sorbent materials for dioxin removal.
I have recently commissioned a similar system which had an equipment arrangement like yours. The convey line length was quite short, relatively speaking, and the negative pressure in the flue duct was actually adequate to convey the sorbent at some of the very low rates.
With just clean air blowing - our upstream pressure of the eductor (IE blower discharge) averaged around 6 psig BUT if you looked at the pressure trend curve in an expanded scale, you can see variations of +/- 0.5 psig happening constantly -- even though the solids feed rate is controlled by a LIW gravimetric feeder.
We had short periods of increasing pressure which we attributed to fluctuations in the discharge of the LIW feeder due to the difference in material head above the LIW screw. When we had a low "refill" set point on the LIW, we saw more fluctuations so we set the refill at 60% and the max fill to 90%. Controlling the refills in that narrower range helped smooth out the discharge and thus the pressure fluctuation. ■
Re: Roots Blower Steady Pressure Increase
Deur Teus,
Some data regarding the kind of material we are injecting:
- Powder activated carbon: Size D50 = 20 micron / Density: 0.5 t/m3
- Limestone: Size D50 = 100 micron / Density: 1.45 t/m3
Regarding the rest of requested data, I can provide an isometric of the pipeline, an assembly drawing of the Venturi device and a general scheme of the installation where you can check location of P1, P2 and Venturi device. Anyway, I have some problems for attaching these files so if you could provide some contact adress I can send them there.
Best regards ■
Re: Roots Blower Steady Pressure Increase
Dear Jack,
Yes, our system intends to reduce dioxins emission at a sintering plant.
The point is that we are not detecting fluctuations in discharge blower pressure but an steady increase of pressure of 0.1 bar along the last 5 months from original value.
In our case, our main problem was the long way between our system and the injection point (about 350 m) that did not allow us to work only by means of main duct suction pressure.
Best regards ■
Re: Roots Blower Steady Pressure Increase
Dear Pablonici
You can send me a message through the forums messaging and e-mail funtion.
Just click on my user name.
Take care
Teus ■
Teus
Roots Blower steady pressure increase
Dear all,
I would like to know your opinion about a phenomenon that we are experiencing at our Active carbon/limestone pneumatic conveying system.
A brief explanation of our system:
- A Roots Blower theoretically designed for a 0,7 bar blower exhaust pressure
- Air from the blower passes through a Venturi eductor device where required amounts of limestone and carbon are injected.
- Materials are conveyed along a 250 m pipeline to the injection point (located at a main collector, and at un under pressure of - 130 mbar)
- We have a pressure transmitter (P1) located at the outlet of the blower and a second one (P2) downstream the Venturi eductor.
- When we started up the system we noticed that pressure measured at P1 was around 0.6-0.65 bar and we set an alarm value for 0.7 bar (pressure relief valve of the blower at 0.8 bar)
- We noticed that P2 value was influenced by injection rate of materials (from 0 to 300 kg/h) but P1 response was hardly constant with slight fluctuations. We understand that main blower discharge pressure is mainly related to Venturi device pressure losses.
- After some months of working, we observed that P1 value started to increase steadily, even achieving alarma value of 0.7 bar. We increased this value but P1 has kept on increasing and nowadays is quite close to 0.75 bar too.
- We though of a possible clogging downstream the Venturi device, but we have dismantled certain pieces of the pipe and, with the pipeline open to the air we have seen that material comes out the pipe at a good velocity, so we think that we can discard material clogging as a possible cause.
So, the point is that discharge pressure of the blower stabilises at a higher value as the time passes by.
We are thinking on the fact that all the causes can be focused at the blower but, can you provide any explanation for this phenomenon? Should we set a higher alarm value (let's say 0.9 bar; of course, all the parts of the system have been designed for supporting this pressure) and fix pressure relief valve at, let´s say, 1 bar in order to be able to work properly? Any relation to discharge temperature of the blower?
Thanks a lot for your help.
Best regards ■