During a material feed, an amount (weight) of raw material or finished product is moved from one location to another. Material feeds are used to control movement of many types of raw or finished products, including powders, granules, solids, slurries, liquids, and even gasses. Material feed management is used in batch, filling and dispensing manufacturing operations.
In filling and dispensing one feed of a raw material or finished product takes place multiple times during a production run. In batch operations multiple feeds of different raw materials and quantities take place in each batch that is completed during a production run.
Why Is Good Material Feed Management Essential to Production?
Good material feed management has a positive impact on productivity for the following reasons:
1. Reduces raw material waste (less cost)
2. Increases batch and filling cycle times (greater line capacity)
3. Increases quality (more effective products)
4. Reduces reworks of finished product (less cost)
5. Reduces finished product loss (less give away)
6. Reduces customer dissatisfaction (fewer complaints of under filled product)
Where Are the Challenges?
There are at least two challenges that need to be considered to successfully managing a material feed.
a) In the real-world process conditions do not stay constant from one feed to the next feed – flow rates vary due to levels in vessels, material consistency, flow characteristics of materials, pump conditions, etc.
b) Communication timing (data transfer update rates) between a distributed field instrument and a PLC or DCS is usually not very deterministic: the time between one read cycle and the next read cycle can vary for many reasons. For example, because of variations in the length of time it takes to send, update, or collect data from buffers, the number of programming loops that must be processed in one program cycle can change from one cycle to the next.
What Measurement and Control Techniques Can Be Used to Address These Challenges?
Following are six techniques that can be deployed to address and solve the above challenges:
1. Stabilize the flow rate (this technique addresses challenge a above). Using this technique usually only possible in liquid or slurry applications. Such as in color kitchens for example. Color kitchens are used to mix up batches of different dyes for fabrics. In this case, each of the raw material vessels is pressurized to a constant pressure to negate the effect of gravity and maintain a constant flow rate no matter what the level (height) of the liquid is in the vessel. Unfortunately, this is a costly solution, and one as said that only applies to some materials.
2. Move the set point comparison as close to the process and measured weight as possible (this technique addresses challenge b above). A deterministic set point comparator (speed and repeatability) plays a big part in delivering a successful material feed result. Moving the set point comparison out of the PLC or DSC into a dedicated scale instrument enables the opportunity for a more deterministic comparison that is close to the process. Remember milliseconds = pounds or grams of material. The higher the flow rate, the higher the potential error.
3. Use historical data from the last feed and slow down the flow rate close to the set point (this technique addresses challenge a above). This is called “adaptive multi-speed feed control.” Until recent times this has been the most common method of addressing the problems. The process flow rate is constrained by brute force, so that any change in flow rate is reduced to a minute amount, thus reducing any potential error in the feed. The set point weight and preact values are sent to the controller and then the preact value is adjusted using the error from the first few feeds to adjust itself to the best preact value. The problem here has always been the cost to install extra pipework, valves, and/or pumps to get control. Secondly there is a cost to manufacturing operations as well, because during every feed we must slow the feed rate down, and this affects and lengthens the batch cycle time.
4. Adapt for the flow variation in real time during every feed (this technique addresses challenge a above). This method is called “adaptive predictive control.” First, an algorithm is used to adapt the pre-act value based on the last feed error, and a second algorithm is used to predict the change needed to the preact in the current feed based on real time flow rate changes monitored during the feed. Depending on how smart the algorithms used are, less severe multi-speed feed control might be able to be used (speeding up feed times). In some cases, single-speed (on/off) feed control can even be used.
5. Move the field instrument into the PLC chassis (this technique addresses challenge b above). This technique takes care of the simplifying or shortening the “communication chain” that the data must travel to be sent to or read from the PLC or DCS. This technique increases determinism and helps to simplify integration time.
6. Use a controller that has physical set point outputs (this technique addresses challenge b above). This techniques offers the fastest method for delivering the STOP signal to the PLC or DSC input.
When should I use these techniques?
It depends on a combination of the following:
• The feed accuracy you are trying to achieve
• The feed rate you are trying to reach
• The batch cycle time operations requires
• The cost of the material you are feeding
• The number of feed done per batch, shift, year
• The material you are feeding
• How the material is being fed
• Whether you are doing GIW or LIW feeds
• The PLC and communications you must use
These are all questions that need to be considered and/or answered. What is worth thinking about is that the more demanding your application is, the more likely you need to use multiple techniques to optimize the material feed management operation/results.
Types of Material Feeds
1. Filling (Gain-in-Weight):
In gain-in-weight (GIW) operations the scale is located on the vessel or container the material is being moved to. GIW feeds are used in filling processes and batch processes (usually when batch cycle time is not critical to operations and it is OK to feed material feeds sequentially into a mixing vessel). Gain-in-weight systems use load points (load cells and mounting hardware) or platform scales to weigh the material being received into a container. An empty container is placed on the scale and its weight is tarred, so that only the net weight of the material going into the container is displayed.
2. Dispensing (Loss-in-Weight):
In loss-in-weight (LIW) operations the scale is installed on the vessel or container the material is being moved from. LIW feeds are used in dispensing processes and batch (when multiple material feeds need to be done into a mixing vessel simultaneously) processes. Supply bins in loss-of-weight applications are suspended by load points to weigh the material being dispensed into receiving containers. Feeders are put on load points or platform scales to weigh the material as it is dispensed.
Rodger Jeffery is vice president of sales, service & marketing, Hardy Process Solutions (San Diego, CA). For more information, email firstname.lastname@example.org, call 858-877-8611, or visit www.hardysolutions.com.