Screw conveyor : Commissioning Guide & Capacity Calculator
How to commission and size a screw feeder ?
Question or remark ? Please contact us at admin@powderprocess.net
| Section summary |
|---|
| 1. Installation qualification (IQ) |
| 2. Operation qualification (OQ) |
| 3. Performance qualification (PQ) |
| 4. Mathematical Equations & Theory |
| 5. Interactive Capacity & Power Calculator |
The commissioning of a screw conveyor or screw feeder must follow very specific steps in order to make sure the equipment is fit for service.
This page gives an overview of key tests to perform on the factory field to commission a screw conveyor through installation qualification, operation qualification, and performance qualification, complemented by an interactive sizing calculator based on CEMA powder handling standard formulas.
1. Installation qualification
A screw conveyor is installed in bulk handling processes to convey and / or dose bulk materials from one area to another, typically to mechanically transport materials from an area of the plant to another, or to dose bulk materials to a scale for example (it is often called a screw feeder in this configuration).
The 1st step is to make sure a particular screw conveyor has been installed properly, even before thinking starting to rotate it for the 1st time.
The following tests are typical of a commissioning checklist of a screw conveyor :
- Installed conveyor model is as per design specification ?
- Model / brand
- Screw flight diameter
- Screw flight pitch
- If possible measure the distance in between the screw flight and the housing
- Inlet and outlet flange diameter
- Material of construction
- Specific design provisions as asked during design, such as quick extraction system of the screw
- Drive type and power
- Dust explosion safety marking (Ex...)
- Upstream and downstream connectors as per process design ?
- Compressed air connected to bearing seal flush (if designed to have it)
- Safety features, such as proximity switch to detect opening, are present
- Optional systems, such as detection of blockage at the end of the screw (often based on a level sensor)
- No foreign body from construction work is inside the valve
2. Operation qualification
Once the Engineer is sure that the correct screw conveyor has been installed, with the correct options and integrated properly to the process, the operation qualification can start. This time, the tests are not anymore static but the equipment will be started. It is however advised to carry out the operation qualification in 2 stages, without product then with product.
Without product :
- Start the screw conveyor at slow speed, maybe 10 Hz on the drive, make sure the direction of rotation is correct and make sure there is no noise (scratching for instance) coming from the rotating screw flight.
- If the previous test is positive, increase the speed until the target speed as per design.
- During the tests, if the screw conveyor is equipped with air pressurization of the bearing seals, make sure the air is turned on and the pressure is correct while the screw rotates (refer to the manufacturer documentation to know the pressure to set).
- Stop the conveyor.
- If it is possible to open the screw, and especially if the valve is equipped with either hatches (common for trough design) or an easy extraction system (for tubular design), then proceed to open the screw conveyor to make sure it is possible as designed (make sure to isolate the power before proceeding). Take the opportunity to inspect the housing as well as the screw flight, to make sure there is no damage after the initial run. Close the conveyor and make sure it can easily be re-assembled as per the manufacturer instructions.
- If the conveyor is equipped with a safety system that is detecting the opening of hatches or covers, make sure it detected the opening and make sure the automation system interlocked the conveyor drive (and possibly other equipment around) as a consequence.
- If the screw conveyor is equipped with a blockage detection system that can detect if material do not flow at the outlet of the screw, test it by simulating a high level.
- Perform any other tests without products required by the process in which the conveyor is integrated (for example run the dosing sequence).
Top 5 Most Popular
1. Pneumatic transport design guide
2. Ribbon blenders
3. Powder mixing
4. Hoppers design guide
5. Measuring degree of mixing
Top 5 New
1. Continuous Dry Mixing
2. Mixing speed
3. Mixer cycle time optimization
4. Batch / continuous mixing comparison
5. Energy Savings
With product :
- Operate the screw conveyor with product. Make sure that the conveyor operation is not affected by the product (no abnormal noise during rotation with product). Check that there is no obvious issues with the material flow (product stuck above the screw, sticking on the screw flight, not discharging properly after the feeder...).
3. Performance qualification
The performance qualification of a screw conveyor is highly linked to the process in which it is integrated. We assume here that the screw conveyor is used to feed a dosing scale Gain in Weight, which is a pretty standard function for this kind of component. The Engineer will have to customize its protocol depending on each of the applications.
- Operate the screw conveyor at nominal speed, make sure the capacity of the conveyor is sufficient vs design.
- Adjust the speed of the conveyor to reach the design capacity and the right dosing accuracy. Adjust coarse dosing speed, fine dosing speed and inflight (common dosing strategy for a Gain in Weight system). Repeat dosing tests several times (for example targeting x kg) to be able to make statistics on the dosing and make sure it is under control.
- Repeat the test for all materials the screw conveyor needs to handle.
- During the tests make sure :
- There is no bridging / ratholing in the hopper above the screw conveyor
- There is no abnormal phenomena such as powder flooding (uncontrolled flow of powder out of the dosing equipment)
- After a series of tests, it is recommended to inspect the inside of the screw (if feasible) to make sure there is no abnormal scratch marks in the screw conveyor.
💡 Industrial Screw Conveyor Commissioning & Design Rules of Thumb
- CEMA Trough Loading Limits: Limit trough loading (\(\phi\)) to 15–30% for abrasive or heavy bulk solids, 30% for non-abrasive medium density powders, and up to 45% for lightweight, free-flowing materials to prevent hanger bearing clogging.
- ATEX & Seal Purge Requirements: Maintain air purge pressure at shaft packing glands 0.5–1.0 bar (7–15 psi) above maximum vessel interior pressure. Interlock air pressure switches with drive motor startup.
- Tip Speed vs. Material Attrition: Keep screw tip speed below 0.6 m/s (120 ft/min) for fragile granules or ATEX Zone 20/21 internal boundary classifications to minimize friction heating and static buildup.
- Dosing Scale Optimization (Gain-In-Weight): Program two-stage drive control (e.g., 50 Hz coarse batching down to 5–10 Hz fine trickle feeding) with calibrated inflight compensation to achieve target accuracy within \(\pm 0.2\%\) to \(\pm 0.5\%\).
- Anti-Flooding Control: For fine, cohesive, or aerated powders (e.g., fluidizing powders like fly ash or cocoa), utilize short pitch (\(P = 0.5D\)) or variable pitch flighting at the inlet to prevent uncontrolled powder flooding.
4. Mathematical Equations & Screw Conveyor Capacity Theory
The theoretical volumetric capacity \(Q_{v,\text{theo}}\) of a screw conveyor depends on the flight cross-sectional area, screw pitch, and rotational speed:
\[ Q_{v,\text{theo}} = 60 \cdot \frac{\pi}{4} \left( D^2 - d^2 \right) \cdot P \cdot N \]Where \(D\) is the outer screw diameter, \(d\) is the shaft diameter, \(P\) is the flight pitch, and \(N\) is the shaft speed in RPM. Accounting for the trough loading factor \(\phi\), actual volumetric throughput \(Q_v\) and mass flow rate \(Q_m\) are calculated as:
\[ Q_v = Q_{v,\text{theo}} \cdot \phi \] \[ Q_m = Q_v \cdot \rho_b \]The mechanical drive motor power \(P_{\text{motor}}\) required to overcome friction and transport material across conveyor length \(L\) is estimated using CEMA standards:
\[ P_{\text{motor}} = \frac{P_{\text{empty}} + \left( \frac{Q_m \cdot L \cdot g \cdot F_m}{3.6 \times 10^6} \right)}{\eta} \]Where \(F_m\) is the material friction factor, \(g = 9.81\text{ m/s}^2\), \(P_{\text{empty}}\) is base mechanical friction loss, and \(\eta\) is total transmission efficiency.