How Much Filling Allowance Should Be Added to Tower Packing Quantity?
After calculating tower packing volume, buyers often face one more practical question:
Should I order exactly the theoretical packing volume, or add some extra quantity?
The DAIER Tower Packing Engineering Assistant includes an input called:
Additional Filling Allowance (%)
This field is used to convert the calculated theoretical packed-bed volume into a preliminary purchase quantity.
The current tool interface shows 3–5% as a typical preliminary allowance, but this should not be treated as a universal rule for every project.
[Use the DAIER Tower Packing Engineering Assistant → https://www.pxdaier.com/tower-packing-engineering-assistant.html]
Theoretical Volume vs Purchase Volume
These two values should not be confused.
Theoretical Packing Volume
This is the geometric volume of the packed section inside the tower.
For a cylindrical bed:
V = π × D² / 4 × H
where:
D = tower internal diameter
H = packing bed height
Purchase Volume
This is the quantity actually ordered after applying the agreed project allowance.
The basic logic is:
Purchase Volume = Theoretical Volume × (1 + Allowance %)
DAIER's existing volume guide already distinguishes calculated tower volume from actual purchasing quantity.
Example: 10 m³ Theoretical Packing Volume
Assume the Engineering Assistant calculates:
Theoretical Packing Volume = 10.00 m³
No Additional Allowance
Purchase quantity:
10.00 m³
3% Allowance
10.00 × 1.03 = 10.30 m³
5% Allowance
10.00 × 1.05 = 10.50 m³
The allowance therefore does not change the physical tower volume.
It changes only the preliminary purchasing basis.
Why Might Additional Packing Be Considered?
Depending on the project, a small additional quantity may be considered for practical reasons such as:
installation handling;
replacement of damaged pieces;
transportation damage;
irregular random-packing handling;
project-specific spare requirements.
DAIER's current tower-packing volume guide already notes that final purchasing quantity can differ from geometric volume for these practical reasons.
But the correct allowance should follow the actual project requirement.
Is 3–5% Always Required?
No.
The 3–5% shown in the DAIER Engineering Assistant is a preliminary planning reference, not a mandatory purchasing rule.
A project may use:
0%;
3%;
5%;
another specified allowance;
depending on its procurement and installation basis.
If the EPC, end user or technical specification already defines the purchase quantity, use that project requirement instead of automatically adding another percentage.
When Should You Enter 0%?
Use 0% additional allowance when:
the required purchase quantity has already been fixed;
the buyer specifically requests the theoretical bed volume only;
the project specification does not permit an additional quantity;
the quotation is being prepared strictly against a stated volume.
For example:
Required Quantity: 15.0 m³ PP Pall Rings
If this is already the approved purchasing quantity, do not automatically turn it into 15.75 m³ simply because the tool offers an allowance field.
Avoid Double-Counting the Allowance
This is one of the most important mistakes to avoid.
Suppose the customer states:
Purchase Quantity: 10.5 m³
and this number already includes a 5% allowance over a 10 m³ theoretical volume.
If you enter:
Packing Volume = 10.5 m³
and then apply another:
5% allowance
the new result becomes:
11.025 m³
The allowance has now been applied twice.
Before entering the field, determine whether the customer's quantity is:
theoretical volume
or
final purchase volume.
Do Not Change Tower Dimensions to Add Allowance
Another mistake is increasing the packing bed height artificially.
For example, if the actual tower packing height is:
3.0 m
do not enter:
3.15 m
simply to create 5% extra packing.
The Engineering Assistant separates these concepts deliberately:
Tower geometry
and
Additional Filling Allowance
should remain separate.
Use the actual tower geometry first.
Apply the purchasing allowance afterward.
Multiple Beds: Apply the Allowance After the Correct Volume Basis
For multiple packing beds, first calculate the correct theoretical volume.
For example:
Bed 1
5.0 m³
Bed 2
6.0 m³
Total theoretical volume:
11.0 m³
If the same 5% purchasing allowance applies to the complete packing scope:
11.0 × 1.05 = 11.55 m³
The important sequence is:
calculate the correct bed volume first
↓
then apply the agreed allowance
The allowance should not be used to correct an incorrect bed-height or number-of-beds input.
Different Packing Products Should Be Kept Separate
Suppose one tower contains:
Bed 1
5 m³ PP Pall Ring
Bed 2
4 m³ ceramic saddle packing
Do not simply combine them into:
9 m³ total packing + 5%
for purchasing.
They are different products and may require separate:
quantities;
allowances;
bulk densities;
weights;
packaging;
quotations.
Calculate and purchase each product on its own basis.
Filling Allowance Is Not Void Fraction
These terms describe completely different concepts.
Void Fraction
Describes the open space inside a packed bed.
Filling Allowance
Describes additional purchase quantity added to the calculated geometric packing volume.
Do not use packing void fraction to reduce the tower packing volume.
If the tower packed section is 10 m³, the random packing requirement is based on approximately that packed-bed volume—not on the solid material volume inside each ring.
Filling Allowance Is Not Bulk Density
Bulk density is used for weight estimation:
Packing Weight = Packing Volume × Bulk Density
The filling allowance modifies the purchase volume.
For example:
Theoretical Volume: 10 m³Allowance: 5%Purchase Volume: 10.5 m³Confirmed Bulk Density: 90 kg/m³
Preliminary purchase weight:
10.5 × 90 = 945 kg
These inputs solve different problems.
DAIER's tool separately asks for Additional Filling Allowance (%) and Confirmed Bulk Density (kg/m³) for this reason.
Should Allowance Be Added Before Weight Calculation?
If the goal is to estimate the weight of the purchase quantity, then weight should be based on the final purchase volume.
The logic is:
Step 1 — Calculate theoretical volume
↓
Step 2 — Apply agreed filling allowance
↓
Step 3 — Obtain preliminary purchase volume
↓
Step 4 — Apply confirmed packing bulk density
↓
Step 5 — Estimate purchase packing weight
This is different from calculating only the installed theoretical bed weight.
Example: Complete Calculation
Assume:
Tower ID: 2.0 mPacking Height: 3.0 mNumber of Beds: 2
Cross-sectional area:
π × 2² / 4 ≈ 3.14 m²
Theoretical packing volume:
3.14 × 3 × 2 ≈ 18.84 m³
If the project applies:
3% additional filling allowance
Purchase volume:
18.84 × 1.03 ≈ 19.41 m³
If the confirmed packing bulk density is:
85 kg/m³
Estimated purchase packing weight:
19.41 × 85 ≈ 1,650 kg
The calculation chain is therefore:
Tower Geometry → Theoretical Volume → Purchase Allowance → Purchase Volume → Packing Weight
What Should Be Written in the RFQ?
Do not send only:
Quantity: 19.41 m³
when the calculation basis is available.
A clearer RFQ can state:
Theoretical Packing Volume: 18.84 m³Additional Purchase Allowance: 3%Preliminary Purchase Quantity: 19.41 m³
Then ask the supplier to:
Please confirm the final quotation quantity based on the project requirement and packing method.
This makes the quantity basis transparent.
For RFQ preparation:
[How to Prepare a Tower Packing RFQ with the DAIER Engineering Assistant → https://www.pxdaier.com/tower-packing-solutions/how-to-prepare-tower-packing-rfq-with-engineering-assistant]
What If the Customer Only Gives a Quantity?
Suppose the inquiry says:
Need 12 m³ Pall Rings.
Do not automatically assume whether the 12 m³ represents:
theoretical tower volume;
final purchase quantity;
quantity including spare allowance.
If tower dimensions are also available, compare the stated quantity with the calculated volume.
If the difference is significant, clarify the basis before changing the quotation quantity.
Replacement Projects Need Extra Care
For an existing tower replacement, the previous purchase quantity may not always equal the current theoretical volume.
Possible reasons include:
historical installation allowance;
remaining spare packing;
different previous packing model;
packing loss during maintenance;
old project purchasing practice.
Use the confirmed tower dimensions as the geometric basis and then determine the new project purchase allowance separately.
For replacement projects:
[How to Use the DAIER Tower Packing Engineering Assistant for an Existing Tower Replacement → INTERNAL LINK]
Quick Guide
Tool calculates 20 m³ and no allowance is specified?
Use 20 m³ as the preliminary geometric basis.
Buyer wants a preliminary 3% allowance?
Purchase volume:
20.6 m³
Customer's 20.6 m³ already includes 3%?
Do not apply another 3%.
EPC specifies exactly 20 m³?
Follow the approved project quantity unless instructed otherwise.
Different packing types in different beds?
Calculate and apply purchase quantities separately.
Not sure whether allowance is already included?
Clarify before final quotation.
The Most Important Rule
The Additional Filling Allowance field should answer one question:
How much extra purchasing quantity should be added after the correct theoretical packed-bed volume has already been calculated?
It should not be used to compensate for:
wrong tower diameter;
wrong packing height;
wrong number of beds;
wrong packing model;
incorrect bulk density.
Get the geometry right first.
Then apply the purchasing allowance.
Use the DAIER Tower Packing Engineering Assistant
The [DAIER Tower Packing Engineering Assistant → https://www.pxdaier.com/tower-packing-engineering-assistant.html] currently separates:
Tower Inside Diameter;
Packing Bed Height;
Number of Beds;
Additional Filling Allowance;
Confirmed Bulk Density;
Theoretical Volume;
Purchase Volume;
Estimated Packing Weight.
This makes it possible to keep:
tower geometry
separate from
purchase allowance
and
weight estimation.
For the basic packing-volume calculation:
[How to Calculate Tower Packing Volume → https://www.pxdaier.com/tower-packing-solutions/calculate-tower-packing-volume]
The best practice is simple:
calculate the real tower volume first → apply the agreed project allowance once → clearly state the final purchase basis.
Specs and test data available upon request.