A permeability number is useful only when its test context is clear
When a mill requests a replacement press felt because of a permeability concern, the most useful starting point is not a single target number. A reading can support a technical discussion only when the mill also records how, where and when it was taken, together with the press position and current operating observations.
This checklist helps a mill prepare factual information for a replacement enquiry or technical review. It does not establish a universal “correct” permeability, diagnose the cause of poor dewatering, or replace the mill's own process and safety decisions.
For the product scope, see press felt products. For the broader design variables that may be reviewed after the machine context is known, see the press felt selection guide.
1. Identify the exact press position and the reason for review
Start the RFQ with the paper machine, press position, paper grade and whether the request is a planned replacement, an urgent review, or a trial discussion. State the normal running conditions that are relevant to the position and note what changed before the concern was observed.
Describe the issue as an observation rather than a conclusion. For example, record when a dewatering concern appeared, whether it is repeated or intermittent, and which machine position is affected. A permeability reading by itself cannot show that the felt is the only cause of a press-section result.
If the mill first needs to assess condition before deciding on replacement, use How to Assess a Press Felt Before Replacement.
2. Record the permeability reading with its method
Provide the value, unit and test method used by the mill or laboratory. Add the instrument or method reference where available, the date of the test, the location or locations sampled, and the felt condition when the reading was taken. If testing was repeated, send the individual readings rather than only an average.
Also record practical context that can affect how the figure is interpreted: whether the felt was in service or removed, whether it had recently been washed or conditioned, its approximate running age, and whether the reading represents a local area or a broader sample. If a value came from an old data sheet rather than a current measurement, label it as historical information.
Do not convert one unit to another or compare values from different methods without stating the method. Clear test context is more useful than an apparently precise number without a source.
3. Give the current felt identity and basic order information
Send the current felt's supplier and product identification when known, its installation date or running age, current dimensions, seam or installation arrangement, and any available approved data sheet. Confirm whether the width, length, running direction and installation access remain the same for the next order.
If the current construction, batt details or base-fabric information are not known, say so. An unknown field should not be converted into an assumed specification. With the mill's permission, clear photos, a previous approved specification, or a sample may provide additional context for a technical review.
4. Document dewatering and conditioning observations separately
Record observations that are relevant to the decision: changes in press-section water handling, sheet condition, cleaning or conditioning practice, vacuum or press settings, grade changes, shutdowns, or maintenance near the time the issue appeared. State the time frame and affected position.
This record helps separate what is known from what still needs checking. It does not prove that a permeability value, a felt construction, or one machine setting caused the result. For a structured review of a specific dewatering concern, see Poor Nip Dewatering: What to Check Before Changing a Press Felt.
5. Include the commercial requirements that affect the RFQ
State the required quantity, destination country, requested delivery window, preferred Incoterm if already decided, and the contact person who can clarify technical questions. For a trial, identify the exact position, review period, acceptance process and any site requirements before an offer is requested.
The purpose is to allow the supplier to state its assumptions and identify missing information before proposing a design. It is not a commitment that a recommendation will deliver a particular machine result.
Field-data checklist for a technical review
| Send to the review team | Why the context matters |
|---|---|
| Machine, press position and paper grade | Links the request to a defined duty rather than a generic felt name. |
| Permeability value, unit, method, date and sample location | Shows what was measured and makes unlike readings easier to distinguish. |
| Current felt identity, age and available approved data | Gives a traceable starting point for comparison. |
| Dimensions, running direction and installation constraints | Helps confirm whether the replacement is physically suitable to review. |
| Factual dewatering and conditioning observations | Identifies the operating context without assuming the cause. |
| Quantity, destination and requested timing | Lets the supplier assess the commercial scope of the enquiry. |
Ask for an assumption-led recommendation
A well-prepared enquiry should invite the supplier to list the information it used, the open questions it still has, and any recommended next checks. That makes the technical discussion auditable for both sides. The mill should validate the final choice through its own engineering, operating and purchasing process.
If you have the field record ready, request a press-felt permeability review. Include only information you are permitted to share.
Frequently asked questions
Is there one “correct” permeability for every press felt?
No. A value needs press position, paper grade, test method, current felt condition and operating context before it can be discussed usefully. This checklist does not set a universal target.
Can a supplier quote from an old felt data sheet?
An old approved data sheet can be useful reference material, but the mill should confirm the current position, dimensions, installation arrangement and reason for replacement. Changes should be identified rather than assumed away.
Does poor dewatering prove that the felt must be replaced?
No. Poor dewatering is an observation that may need a structured review of the press position and operating context. Use the poor nip dewatering checklist before drawing a conclusion.
