Wearing A Purdah While In Ihraam

If she observes purdah in such a way that the cloth of the Niqaab/veil does not touch her face at all(for example by placing some cap on her head and letting the veil come down in front of her faceover the cap in a manner that it does not touch the face), no compensation or penalty will be due.

However, if purdah is observed in a way that the cloth of the veil touches her face and this is for a minimal period like 20 minutes as queried by you, then she will be required to discharge some Sadaqah. The Sadaqah in this case will be a handful of wheat as it is less than an hour. If the face was covered in this way for a complete day then Damm will be necessary. Similarly, if the female in Ihraam intends covering her face in this way for several days, then as long as she intends continuing the practice of covering her face, meaning that when she removes her Niqaab in the evening she intends adorning it the next full day as well, then she will be required to discharge only a single Damm at the end of her Ihraam period.