SFM | Solid Flow Monitor | BBD

A solid flow monitor, also known as BBD or broken bag detector, is a type of sensor that is installed in the return duct of a fluid bed dryer, fluid bed processor, or fluid bed equipment.

In this article, we will discuss the following,

  • What Is BBD or SFM?
  • How does SFM or BBD work?
  • Functions Of Solid Flow Monitor?
  • Advantages Of SFM?
  • How Is the Challenge Test for BBD Performed?
  • Frequency Of Challenge Test?

What Is BBD or SFM?

A sensor which is installed in the Exhaust duct of FBD, FBP, or FBE is known as SFM or solid flow monitor, or solid flow meter.

BBD is the other name of the same sensing probe which is a Broken Bag Detector.

Filter sense is also another name which is used for same detection system.

Working Principle Of SFM

BBD or SFM works on a very basic principle of detection, where it sends signals to the system, to ring the alarm & shut down the dryer whenever solid particles flow is detected in the return air duct of the fluid bed dryer.

Functions Of Solid Flow Monitor

The installation of BBD of SFM in the fluid bed dryer is of great importance because its function is to detect the flow of solid material due to damage to fluid bed dryer filters or due to any pinhole present in the filters of fluid bed dryer, fluid bed processor or fluid bed equipment.

On detecting the flow of solid particles this sensor shut down the dryer with an alarm indication on HMI.


As we know that a fluid bed dryer works on the principle of fluidization, where our product is lifted, come down & again lifted until the required level of LOD is not achieved.

The product is retained inside the drying chamber with the help of filters which allow air to pass & retain powered materials inside.

In normal conditions, the retention of product is ensured by the filters but if there is any damage to the filters, or there is a pinhole in any of the filter’s pockets,it will allow powders to go outside along with exhaust air.

It is a very alarming condition because product loss will occur, to prevent this product loss a BBD or solid flow monitoring system is installed in the return air duct of the fluid bed dryer which helps to prevent product loss by shutting down the fluid bed dryer.

Advantages Of SFM?

The following are some main advantages of installing a Broken Bag detector in the return air duct of the dryer,

Prevent Product Loss

As we discussed earlier the main advantage of BBD or SFM is to prevent product loss.

Environmental Safety 

Spillage of powder particles from FBD may pollute the environment so by preventing the leakage of powder particles, the environment remains safe.

Equipment Safety 

The leakage of powder particles & their accumulation may affect the working & efficiency of the exhaust filters & blower which is installed at the end of the exhaust air duct.

So turning the FBD off on detection of powder leaking,the life of downstream equipment is increased.

Potent Products

Loss of potent drugs to the environment may be very harmful to the surrounding population so by SFM the loss of potent drugs can be controlled.

How Is the Challenge Test for BBD Performed?

To check whether our BBD or solid flow monitoring system is working properly or not we must change the SFM or BBD sensor.


For the challenge test of the solid flow meter, we must have 100 to 200 grams of any diluent that is present in the formulation of the next batch which we are going to manufacture after the challenge test.


Some people specify that we should only use starch for the challenge test but the drawback is that if we use starch & our next product does not contain starch in its formulation, again we will have to go for type B cleaning which will be nothing but just a waste of time.


  • Suppose after manufacturing 5 batches of product A we have performed major cleaning for product B.
  • Our product B contains Lactose as a diluent so will generate an extra material requirement sheet of 100 to 200 grams of Lactose for the challenge test.
  • If we use Lactose for the challenge test there is no need to perform major cleaning after the challenge test as our next product contains the same Excipient.


After manufacturing, perform major cleaning and ensure that FBD & sensor probe is properly cleaned & washed.

Pull the product trolley out & remove the filters of FBD.

Take 100 to 200 grams of material which is dispensed for a challenge test & spread it over the Dutch Woven Screen of the Fluid bed dryer.

Lock the product trolley in the drying chamber & turn FBD on according to SOP.

When fluidization occurs, the spread material (lactose) will be blown out along with exhaust air.

As soon as the material is detected by the BBD or filter sense , the dryer must be shut down in 2 to 3 seconds with an alarm system.

Acceptance Criteria

If the FBD, FBP, or FBE is shut down in a specified period it means that our BBD or SFM sensor is working properly & the test is passed.

If FBD, FBP, or FBE is not shut down, it means the bag sensor is not working properly & we should not start manufacturing until the issue is not resolved by the Engineering department.


The frequency of challenge tests can be adjusted according to in-house SOP.

Some companies perform this test weekly & some every month.

The most reliable period to set challenge test frequency is to perform it after each product changeover.

Read More 

  1. Critical Process Parameters Of FBD 
  2. Kevlar Rope Of FBD 
  3. Raking In FBD 
  4. Types Of  Granulation